,worlds ,within ,worlds3 ,the ,story of ,nuclear ,energy ,volume #a ,atomic ,weights ,energy ,electricity by ,isaac ,asimov ,u4 ,s4 ,energy ,research and ,development ,administration ,office of ,public ,affairs ,washington1 ,d4,c4 #bjede ,library of ,congress ,catalog ,card ,number3 #ge-ahidgg #aigb ,nothing in the history of mankind has opened our eyes to the possibilities of science as has the development of atomic power4 ,in the last #bjj years1 people have seen the coming of the steam engine1 the steamboat1 the railroad locomotive1 the automobile1 the airplane1 radio1 motion pictures1 television1 the machine age in general4 ,yet none of it seemed quite so fantastic1 quite so unbelievable1 as what man has done since #aici with the atom 444 there seem to be almost no limits to what may lie ahead3 inexhaustible energy1 new worlds1 ever-widening knowledge of the physical universe4 ,isaac ,asimov ,illustration3 ,photograph of night sky ,the ,u4 ,s4 ,energy ,research and ,development ,administration publishes a series of booklets for the general public4 ,please write to the following address for a title list or for information on a specific subject3 ,,userda--,technical ,information ,center ,p4 ,o4 ,box #fb ,oak ,ridge1 ,tennessee #cghcj ,illustration3 ,isaac ,asimov ,,isaac ,,asimov received his academic degrees from ,columbia ,university and is ,associate ,professor of ,biochemistry at the ,boston ,university ,school of ,medicine4 ,he is a prolific author who has written over #aej books in the past #bj years1 including about #bj science fiction works1 and books for children4 ,his many excellent science books for the public cover subjects in mathematics1 physics1 astronomy1 chemistry1 and biology1 such as ,the ,genetic ,code1 ,inside the ,atom1 ,building ,blocks of the ,universe1 ,understanding ,physics1 ,the ,new ,intelligent ,man's ,guide to ,science1 and ,asimov's ,biographical ,encyclopedia of ,science and ,technology4 ,in #aife ,dr4 ,asimov received the ,james ,t4 ,grady ,award of the ,american ,chemical ,society for his major contribution in reporting science progress to the public4 ,illustration3 ,photograph of night sky ,,volume #a ,introduction #e ,atomic ,weights #f ,electricity #aa ,units of ,electricity #aa ,cathode ,rays #ac ,radioactivity #ag ,the ,structure of the ,atom #be ,atomic ,numbers #cj ,isotopes #ce ,energy #dg ,the ,law of ,conservation of ,energy #dg ,chemical ,energy #ej ,electrons and ,energy #ed ,the ,energy of the ,sun #ee ,the ,energy of ,radioactivity #eg ,,volume #b ,mass and ,energy #fi ,the ,structure of the ,nucleus #ge ,the ,proton #ge ,the ,proton-,electron ,theory #gf ,protons in ,nuclei #hj ,nuclear ,bombardment #hb ,particle ,accelerators #hf ,the ,neutron #ib ,nuclear ,spin #ib ,discovery of the ,neutron #ie ,the ,proton-,neutron ,theory #ih ,the ,nuclear ,interaction #aja ,neutron ,bombardment #ajg ,,volume #c ,nuclear ,fission #aag ,new ,elements #aag ,the ,discovery of ,fission #abb ,the ,nuclear ,chain ,reaction #abg ,the ,nuclear ,bomb #aca ,nuclear ,reactors #ada ,nuclear ,fusion #adg ,the ,energy of the ,sun #adg ,thermonuclear ,bombs #adi ,controlled ,fusion #aea ,beyond ,fusion #aei ,antimatter #aei ,the ,unknown #afd ,reading ,list #aff ,illustration3 ,a total eclipse of the sun4 ,,introduction ,in a way1 nuclear energy has been serving man as long as he has existed4 ,it has served all of life2 it has flooded the earth for billions of years4 ,the sun1 you see1 is a vast nuclear engine1 and the warmth and light that the sun radiates is the product of nuclear energy4 ,in order for man to learn to produce and control nuclear energy himself1 however 7something that did not take place until this century71 three lines of investigation--atoms1 electricity1 and energy--had to develop and meet4 ,we will begin with atoms4 ,,atomic ,,weights ,as long ago as ancient ,greek times1 there were men who suspected that all matter consisted of tiny particles which were far too small to see4 ,under ordinary circumstances1 they could not be divided into anything smaller1 and they were called 8atoms0 from a ,greek word meaning 8indivisible04 ,it was not until #ahjh1 however1 that this 8atomic theory0 was really put on a firm foundation4 ,in that year the ,english chemist ,john ,dalton 7#agff-ahdd7 published a book in which he discussed atoms in detail4 ,every element1 he suggested1 was made up of its own type of atoms4 ,the atoms of one element were different from the atoms of every other element4 ,the chief difference between the various atoms lay in their mass1 or weight4#a ,dalton was the first to try to determine what these masses might be4 ,he could not work out the actual masses in ounces or grams1 for atoms were far too tiny to weigh with any of his instruments4 ,he could1 however1 determine their relative weights2 that is1 how much more massive one kind of atom might be than another4 ,for instance1 he found that a quantity of hydrogen gas invariably combined with eight times its own mass of oxygen gas to form water4 ,he guessed that water consisted of combinations of #a atom of hydrogen with #a atom of oxygen4 7,a combination of atoms is called a 8molecule0 from a ,greek word meaning 8a small mass01 and so hydrogen and oxygen atoms can be said to combine to form a 8water molecule047 ,illustration3 ,john ,dalton ,to account for the difference in the masses of the combining gases1 ,dalton decided that the oxygen atom was eight times as massive as the hydrogen atom4 ,if he set the mass of the hydrogen atom at #a 7just for convenience7 then the mass of the oxygen atom ought to be set at #h4 ,these comparative1 or relative1 numbers were said to be 8atomic weights01 so that what ,dalton was suggesting was that the atomic weight of hydrogen was #a and the atomic weight of oxygen was #h4 ,by noting the quantity of other elements that combined with a fixed mass of oxygen or of hydrogen1 ,dalton could work out the atomic weights of these elements as well4 ,dalton's idea was right1 but his details were wrong in some cases4 ,for instance1 on closer examination it turned out that the water molecule was composed of #a oxygen atom and #b hydrogen atoms4 ,for this reason1 the water molecule may be written ,h,o1 where ,h is the chemical symbol for a hydrogen atom1 and ,o for an oxygen atom4 ,it is still a fact that a quantity of hydrogen combines with eight times its mass of oxygen1 so the single oxygen atom must be eight times as massive as the #b hydrogen atoms taken together4 ,the oxygen atom must therefore be sixteen times as massive as a single hydrogen atom4 ,if the atomic weight of hydrogen is #a1 then the atomic weight of oxygen is #af4 ,at first it seemed that the atomic weights of the various elements were whole numbers and that hydrogen was the lightest one4 ,it made particular sense1 then1 to consider the atomic weight of hydrogen as #a1 because that made all the other atomic weights as small as possible and therefore easy to handle4 ,the ,swedish chemist ,jns ,jakob ,berzelius 7#aggi-ahdh7 continued ,dalton's work and found that elements did not combine in quite such simple ratios4 ,a given quantity of hydrogen actually combined with a little bit less than eight times its mass of oxygen4 ,therefore if the atomic weight of hydrogen were considered to be #a1 the atomic weight of oxygen would have to be not #af1 but #ae4hg4 ,illustration3 ,jns ,jakob ,berzelius ,as it happens1 oxygen combines with more elements 7and more easily7 than hydrogen does4 ,the ratio of its atomic weight to that of other elements is also more often a whole number4 ,in working out the atomic weight of elements it was therefore more convenient to set the atomic weight of oxygen at a whole number than that of hydrogen4 ,berzelius did this1 for instance1 in the table of atomic weights he published in #ahbh4 ,at first he called the atomic weight of oxygen #ajj4 ,then he decided to make the atomic weights as small as possible1 without allowing any atomic weight to be less than #a4 ,for that reason1 he set the atomic weight of oxygen at exactly #af and in that case1 the atomic weight of hydrogen had to be placed just a trifle higher than #a4 ,the atomic weight of hydrogen became #a4jjh4 ,this system was retained for nearly a century and a half4 ,throughout the #aith century1 chemists kept on working out atomic weights more and more carefully4 ,by the start of the #bjth century1 most elements had their atomic weights worked out to two decimal places1 sometimes three4 ,a number of elements had atomic weights that were nearly whole numbers on the 8oxygen #af0 standard4 ,the atomic weight of aluminum was just about #bg1 that of calcium almost #dj1 that of carbon almost #ab1 that of gold almost #aig1 and so on4 ,on the other hand1 some elements had atomic weights that were far removed from whole numbers4 ,the atomic weight of chlorine was close to #ce4e1 that of copper to #fc4e1 that of iron to #ee4h1 that of silver to #ajg4i1 and so on4 ,throughout the #aith century1 chemists did not know why so many atomic weights were whole numbers1 while others weren't4 ,they simply made their measurements and recorded what they found4 ,for an explanation1 they had to wait for a line of investigation into electricity to come to fruition4 ,,electricity ,units of ,electricity ,through the #ahth century1 scientists had been fascinated by the properties of electricity4 ,electricity seemed1 at the time1 to be a very fine fluid that could extend through ordinary matter without taking up any room4 ,electricity did more than radiate through matter1 however4 ,it also produced important changes in matter4 ,in the first years of the #aith century1 it was found that a current of electricity could cause different atoms or different groups of atoms to move in opposite directions through a liquid in which they were dissolved4 ,the ,english scientist ,michael ,faraday 7#agia-ahfg7 noted in #ahcb that a given quantity of electricity seemed to liberate the same number of atoms of a variety of different elements4 ,in some cases1 though1 it liberated just half the expected number of atoms2 or even1 in a few cases1 just a third4 ,scientists began to speculate that electricity1 like matter1 might consist of tiny units4 ,when electricity broke up a molecule1 perhaps a unit of electricity attached itself to each atom4 ,in that case1 the same quantity of electricity1 containing the same number of units1 would liberate the same number of atoms4 ,in the case of some elements1 each atom could attach #b units of electricity to itself1 or perhaps even #c4 ,when that happened a given quantity of electricity would liberate only one-half1 or only one-third1 the usual number of atoms4 7,thus1 #ah units of electricity would liberate #ah atoms if distributed #a to an atom2 only #i atoms if distributed #b to an atom2 and only #f atoms if distributed #c to an atom47 ,it was understood at the time that electricity existed in two varieties1 which were called positive and negative4 ,it appeared that if an atom attached a positive unit of electricity to itself it would be pulled in one direction through the solution by the voltage4 ,if it attached a negative unit of electricity to itself it would be pulled in the other direction4 ,illustration3 ,michael ,faraday ,the units of electricity were a great deal more difficult to study than the atomic units of matter1 and throughout the #aith century they remained elusive4 ,in #ahia1 though1 the ,irish physicist ,george ,johnstone ,stoney 7#ahbf-aiaa7 suggested that the supposed unit of electricity be given a name at least4 ,he called the unit an 8electron04 ,cathode ,rays ,an electric current flows through a closed circuit of some conducting material1 such as metal wires4 ,it starts at one pole of a battery1 or of some other electricity generating device1 and ends at the other4 ,the two poles are the positive pole or 8anode0 and the negative pole or 8cathode04 ,if there is a break in the circuit1 the current will usually not flow at all4 ,if1 however1 the break is not a large one1 and the current is under a high driving force 7which is called the 8voltage071 then the current may leap across the break4 ,if two ends of a wire1 making up part of a broken circuit1 are brought close to each other with nothing but air between1 a spark may leap across the narrowing gap before they actually meet and1 while it persists1 the current will flow despite the break4 ,the light of the spark1 and the crackling sound it makes1 are the results of the electric current interacting with molecules of air and heating them4 ,neither the light nor the sound is the electricity itself4 ,in order to detect the electricity1 the current ought to be forced across a gap containing nothing1 not even air4 ,in order to do that1 wires would have to be sealed into a glass tube from which all 7or almost all7 the air was withdrawn4 ,this was not easy to do and it was not until #ahed that ,heinrich ,geissler 7#ahad-ahgi71 a ,german glass-blower and inventor1 accomplished this feat4 ,the wires sealed into such a 8,geissler tube0 could be attached to the poles of an electric generator1 and if enough voltage was built up1 the current would leap across the vacuum4 ,illustration3 ,a ,geissler tube4 labelled3 ,current source ,such experiments were first performed by the ,german physicist ,julius ,plcker 7#ahja-ahfh74 ,in #aheh he noticed that when the current flowed across the vacuum there was a greenish glow about the wire that was attached to the cathode of the generator4 ,others studied this glow and finally the ,german physicist ,eugen ,goldstein 7#ahej-aica7 decided in #ahgf that there were rays of some sort beginning at the wire attached to the negatively charged cathode and ending at the part of the tube opposite the cathode4 ,he called them 8cathode rays04 ,these cathode rays1 it seemed1 might well be the electric current itself1 freed from the metal wires that usually carried it4 ,if so1 determining the nature of the cathode rays might reveal a great deal about the nature of the electric current4 ,were cathode rays something like light and were they made up of tiny waves8 ,or were they a stream of particles possessing mass8 ,there were physicists on each side of the question4 ,by #ahhe1 however1 the ,english physicist ,william ,crookes 7#ahcb-aiai7 showed that cathode rays could be made to turn a small wheel when they struck that wheel on one side4 ,this seemed to show that the cathode rays possessed mass and were a stream of atom-like particles1 rather than a beam of mass-less light4 ,furthermore1 ,crookes showed that the cathode rays could be pushed sideways in the presence of a magnet4 7,this effect1 when current flows in a wire1 is what makes a motor work47 ,this meant that1 unlike either light or ordinary atoms1 the cathode rays carried an electric charge4 ,illustration3 ,j4 ,j4 ,thomson in his laboratory4 ,on his right are early ,x-ray pictures4 ,this view of the cathode rays as consisting of a stream of electrically charged particles was confirmed by another ,english physicist1 ,joseph ,john ,thomson 7#ahef-aidj74 ,in #ahig he showed that the cathode rays could also be made to take a curved path in the presence of electrically charged objects4 ,the particles making up the cathode rays were charged with negative electricity1 judging from the direction in which they were made to curve by electrically charged objects4 ,thomson had no hesitation in maintaining that these particles carried the units of electricity that ,faraday's work had hinted at4 ,eventually1 ,stoney's name for the units of electricity was applied to the particles that carried those units4 ,the cathode rays1 in other words1 were considered to be made up of streams of electrons and ,thomson is usually given credit for having discovered the electron4 ,the extent to which cathode rays curved in the presence of a magnet or electrically charged objects depended on the size of the electric charge on the electrons and on the mass of the electrons4 ,ordinary atoms could be made to carry an electric charge and by comparing their behavior with those of electrons1 some of the properties of electrons could be determined4 ,there were1 for instance1 good reasons to suppose that the electron carried a charge of the same size as one that a hydrogen atom could be made to carry4 ,the electrons1 however1 were much easier to pull out of their straight-line path than the charged hydrogen atom was4 ,the conclusion drawn from this was that the electron had much less mass than the hydrogen atom4 ,thomson was able to show1 indeed1 that the electron was much lighter than the hydrogen atom1 which was the lightest of all the atoms4 ,nowadays we know the relationship quite exactly4 ,we know that it would take #ahcg4aa electrons to possess the mass of a single hydrogen atom4 ,the electron is therefore a 8subatomic particle02 the first of this sort to be discovered4 ,in #ahig1 then1 two types of mass-containing particles were known4 ,there were the atoms1 which made up ordinary matter1 and the electrons1 which made up electric current4 ,radioactivity ,was there a connection between these two sets of particles--atoms and electrons8 ,in #ahig1 when the electron was discovered1 a line of research that was to tie the two kinds of particles together had already begun4 ,in #ahie the ,german physicist ,wilhelm ,konrad ,roentgen 7#ahde-aibc7 was working with cathode rays4 ,he found that if he made the cathode rays strike the glass at the other end of the tube1 a kind of radiation was produced4 ,this radiation was capable of penetrating glass and other matter4 ,roentgen had no idea as to the nature of the radiation1 and so called it 8,x rays04 ,this name1 containing 8,x0 for 8unknown01 was retained even after physicists worked out the nature of ,x rays and found them to be light-like radiation made up of waves much shorter than those of ordinary light4 ,illustration3 ,antoine ,henri ,becquerel4 ,at once1 physicists became fascinated with ,x rays and began searching for them everywhere4 ,one of those involved in the search was the ,french physicist ,antoine ,henri ,becquerel 7#aheb-aijh74 ,a certain compound1 potassium uranyl sulfate1 glowed after being exposed to sunlight and ,becquerel wondered if this glow1 like the glow on the glass in ,roentgen's ,x-ray tube1 contained ,x rays4 ,illustration3 ,roentgen's laboratory ,illustration3 ,wilhelm ,roentgen and his laboratory at the ,university of ,wrzburg4 ,it did1 but while investigating the problem in #ahif1 ,becquerel found that the compound was giving off invisible penetrating ,x-ray-like radiation continually1 whether it was exposed to sunlight or not4 ,the radiation was detected because it would fog a photographic plate just as light would4 ,what's more1 the radiation would fog the plate1 even if the plate were wrapped in black paper1 so that it could penetrate matter just as ,x rays could4 ,others1 in addition to ,becquerel1 were soon investigating the new phenomenon4 ,in #ahih the ,polish 7later ,french7 physicist ,marie ,sklodowska ,curie 7#ahfg-aicd7 showed that it was the uranium atom that was the source of the radiation1 and that any compound containing the uranium atom would give off these penetrating rays4 ,until then1 uranium had not been of much interest to chemists4 ,it was a comparatively rare metal that was first discovered in #aghi by the ,german chemist ,martin ,heinrich ,klaproth 7#agdc-ahag74 ,it had no particular uses and remained an obscure element4 ,as chemists learned to work out the atomic weights of the various elements1 they found1 however1 that1 of the elements then known1 uranium had the highest atomic weight of all--#bch4 ,once uranium was discovered to be an endless source of radiation1 it gained interest that has risen ever since4 ,madame ,curie gave the name 8radioactivity0 to this phenomenon of continuously giving off rays4 ,uranium was the first element found to be radioactive4 ,it did not remain alone1 however4 ,it was soon shown that thorium was also radioactive4 ,thorium1 which had been discovered in #ahbi by ,berzelius1 was made up of atoms that were the second most massive known at the time4 ,thorium's atomic weight is #bcb4 ,but what was the mysterious radiation emitted by uranium and thorium8 ,almost at once it was learned that whatever the radiation was1 it was not uniform in properties4 ,in #ahii ,becquerel 7and others7 showed that1 in the presence of a magnet1 some of the radiation swerved in a particular direction4 ,later it was found that a portion of it swerved in the opposite direction4 ,still another part didn't swerve at all but moved on in a straight line4 ,the conclusion was that uranium and thorium gave off three kinds of radiation4 ,one carried a positive charge of electricity1 one a negative charge1 and one no charge at all4 ,the ,new ,zealand-born physicist ,ernest ,rutherford 7#ahga-aicg7 called the first two kinds of radiation 8alpha rays0 and 8beta rays01 after the first two letters of the ,greek alphabet4 ,the third was soon called 8gamma rays0 after the third letter4 ,illustration3 ,ernest ,rutherford ,illustration3 ,marie ,curie and her two daughters1 ,eve 7left7 and ,irene1 in #aijh4 ,illustration3 ,pierre ,curie during a class lecture in #aijf1 the year of his death4 ,the gamma rays eventually turned out to be another light-like form of radiation1 with waves even shorter than those of ,x rays4 ,the alpha rays and beta rays1 which carried electric charges1 seemed to be streams of charged particles 78alpha particles0 and 8beta particles07 just as the cathode rays had turned out to be4 ,in #aijj1 indeed1 ,becquerel studied the beta particles and found them to be identical in mass and charge with electrons4 ,they were electrons4 ,by #aijf ,rutherford had worked out the nature of the alpha particles4 ,they carried a positive electric charge that was twice as great as the electron's negative charge4 ,if an electron carried a charge that could be symbolized as -1 then the charge of the alpha particle was 4 ,furthermore1 the alpha particle was much more massive than the electron4 ,it was1 indeed1 as massive as a helium atom 7the second lightest known atom7 and four times as massive as a hydrogen atom4 ,nevertheless1 the alpha particle can penetrate matter in a way in which atoms cannot1 so that it seems much smaller in diameter than atoms are4 ,the alpha particle1 despite its mass1 is another subatomic particle4 ,here1 then1 is the meeting point of electrons and of atoms--the particles of electricity and of matter4 ,ever since ,dalton had first advanced the atomic theory over a century earlier1 chemists had assumed that atoms were the fundamental units of matter4 ,they had assumed atoms were as small as anything could be and that they could not possibly be broken up into anything smaller4 ,the discovery of the electron1 however1 had shown that some particles1 at least1 might be far smaller than any atom4 ,then1 the investigations into radioactivity had shown that atoms of uranium and thorium spontaneously broke up into smaller particles1 including electrons and alpha particles4 ,it would seem1 then1 that atoms of these elements and1 presumably1 of all elements1 were made up of still smaller particles and that among these particles were electrons4 ,the atom had a structure and physicists became interested in discovering exactly what that structure was4 ,the ,structure of the ,atom ,since radioactive atoms gave off either positively charged particles or negatively charged particles1 it seemed reasonable to assume that atoms generally were made up of both types of electricity4 ,furthermore1 since the atoms in matter generally carried no charge at all1 the normal 8neutral atom0 must be made up of equal quantities of positive charge and negative charge4 ,it turned out that only radioactive atoms1 such as those of uranium and thorium1 gave off positively charged alpha particles4 ,many atoms1 however1 that were not radioactive1 could be made to give off electrons4 ,in #ahii ,thomson showed that certain perfectly normal metals with no trace of radioactivity gave off electrons when exposed to ultraviolet light4 7,this is called the 8photoelectric effect047 ,it was possible to suppose1 then1 that the main structure of the atom was positively charged and generally immovable1 and that there were also present light electrons1 which could easily be detached4 ,thomson had suggested1 as early as #ahih1 that the atom was a ball of matter carrying a positive charge and that individual electrons were stuck throughout its substance1 like raisins in pound cake4 ,if something like the ,thomson view were correct then the number of electrons1 each with one unit of negative electricity1 would depend on the total size of the positive charge carried by the atom4 ,if the charge were #e1 there would have to be #e electrons present to balance that4 ,the total charge would then be #j and the atom as a whole would be electrically neutral4 ,if1 in such a case1 an electron were removed1 the atomic charge of #e would be balanced by only #d electrons with a total charge of -#d4 ,in that case1 the net charge of the atom as a whole would be #a4 ,on the other hand1 if an extra electron were forced onto the atom1 the charge of #e would be balanced by #f electrons with a total charge of -#f1 and the net charge of the atom as a whole would be -#a4 ,such electrically charged atoms were called 8ions0 and their existence had been suspected since ,faraday's day4 ,faraday had known that atoms had to travel through a solution under the influence of an electric field to account for the way in which metals and gases appeared at the cathode and anode4 ,it was he who first used the term1 ion1 from a ,greek word meaning 8traveller04 ,the word had been suggested to him by the ,english scholar1 ,william ,whewell 7#agid-ahff74 ,in #ahhd the ,swedish chemist ,svante ,august ,arrhenius 7#ahei-aibg7 had first worked out a detailed theory based on the suggestion that these ions were atoms or groups of atoms that carried an electric charge4 ,illustration3 ,svante ,a4 ,arrhenius ,by the close of the #aith century1 then1 ,arrhenius's suggestion seemed correct4 ,there were positive ions made up of atoms or groups of atoms1 from which one or more of the electrons within the atoms had been removed4 ,there were negative ions made up of single atoms or of groups of atoms1 to which one or more extra electrons had been added4 ,illustration3 ,neutral atom ,each unit of positive charge is balanced by a unit of negative charge ,in this case1 total charge #b -#b #j ,ionized atom3 ,if an electron is removed1 the balance is destroyed ,in this case1 total charge #b -#a #a ,although ,thomson's model of the atom explained the existence of ions and the fact that atoms could give off electrons or absorb them1 it was not satisfactory in all ways4 ,further investigations yielded results not compatible with the raisins-in-the-pound-cake notion4 ,in #aijf ,rutherford began to study what happened when massive subatomic particles1 such as alpha particles1 passed through matter4 ,when alpha particles passed through a thin film of gold1 for instance1 they raced through1 for the most part1 as though nothing were there4 ,the alpha particles seemed to push the light electrons aside and to act as though the positively charged main body of the atom that ,thomson had pictured was not solid1 but was soft and spongy4 ,the only trouble was that every once in a while an alpha particle seemed to strike something in the gold film and bounce to one side4 ,sometimes it even bounced directly backward4 ,it was as though somewhere in each atom there was something at least as massive as the alpha particle4 ,how large was this massive portion of the atom8 ,it couldn't be very large for if it were the alpha particles would hit it frequently4 ,instead1 the alpha particles made very few hits4 ,this meant the massive portion was very small and that most alpha particles tore through the atom without coming anywhere near it4 ,illustration3 ,rutherford's alpha particle bombardment apparatus4 ,a piece of radium in the lead box 7,b7 emits alpha particles that go through the gold foil 7,f74 ,these particles are scattered at different angles onto the fluorescent screen 7,s71 where the flashes caused by each impact are seen through the microscope 7,m74 ,below1 alpha particles are shown bouncing off a nucleus in the gold foil4 ,illustration3 ,by #aiaa ,rutherford announced his results to the world4 ,he suggested that just about all the mass of the atom was concentrated into a very tiny1 positively charged 8nucleus0 at its center4 ,the diameter of the nucleus was only about #a_/aj1jjj the diameter of the atom4 ,all the rest of the atom was filled with the very light electrons4 ,illustration3 ,hans ,geiger 7left7 and ,ernest ,rutherford at ,manchester ,university about #aiaj4 ,according to ,rutherford's notion1 the atom consisted of a single tiny positively charged lead shot at the center of a foam of electrons4 ,it was ,thomson's notion in reverse4 ,still1 the nucleus carried a positive charge of a particular size and was balanced by negatively charged electrons4 ,rutherford's model of the atom explained the existence of ions just as easily as ,thomson's did and it explained more besides4 ,for instance1 if all the electrons are removed so that only the nucleus remains1 this nucleus is as massive as an atom but is so tiny in size that it can penetrate matter4 ,the alpha particle would be a bare atomic nucleus from this point of view4 ,rutherford's model of the 8nuclear atom0 is still accepted today4 ,atomic ,numbers ,since the atom consisted of a positively charged nucleus at the center1 and a number of negatively charged electrons outside1 the next step was to find the exact size of the nuclear charge and the exact number of electrons for the different varieties of atoms4 ,the answer came through a line of research that began with the ,english physicist ,charles ,glover ,barkla 7#ahgg-aidd74 ,in #aiaa he noted that when ,x rays passed through atoms1 some were absorbed and some bounced back4 ,those that bounced back had a certain ability to penetrate other matter4 ,when the ,x rays struck atoms of high atomic weight1 the ,x rays that bounced back were particularly penetrating4 ,in fact1 each different type of atom seemed associated with reflected ,x rays of a particular penetrating power1 so ,barkla called these 8characteristic ,x rays04 ,in #aiac another ,english physicist1 ,henry ,gwyn-,jeffreys ,moseley 7#ahhg-aiae71 went into the matter more thoroughly4 ,he measured the exact wavelength of the characteristic ,x rays by reflecting them from certain crystals4 ,in crystals1 atoms are arranged in regular order and at known distances from each other4 ,x rays reflecting from 7or more accurately1 diffracting from7 crystals are bent out of their path by the rows of atoms4 ,the longer their waves1 the more they are bent4 ,from the degree of bending the wavelength of the waves can be determined4 ,illustration3 ,charles ,glover ,barkla ,illustration3 ,henry ,gwyn-,jeffreys ,moseley ,moseley found that the greater the atomic weight of an atom1 the shorter the waves of the characteristic ,x rays associated with it and the more penetrating those ,x rays were4 ,there was such a close connection1 in fact1 that ,moseley could arrange the elements in order according to the wavelength of the characteristic ,x rays4 ,for some #dj years prior to this1 the elements had been listed in order of atomic weight4 ,this was useful especially since the ,russian chemist ,dmitri ,i4 ,mendelev 7#ahcd-aijg7 had arranged them in a 8periodic table0 based on the atomic weight order in such a way that elements of similar properties were grouped together4 ,the elements in this table were sometimes numbered consecutively 78atomic number07 but this was inconvenient since1 when new elements were discovered1 the list of atomic numbers might have to be reorganized4 ,illustration3 ,dmitri ,mendelev and ,bohuslav ,brauner in ,prague in #aijj4 ,brauner was a professor of chemistry at the ,bohemian ,university in ,prague4 ,the ,danish physicist ,niels ,bohr 7#ahhe-aifb7 had just advanced a theory of atomic structure that made it reasonable to suppose that the wavelength of the characteristic ,x rays depended on the size of the nuclear charge of the atoms making up a particular element4 ,moseley therefore suggested that these ,x rays be used to determine the size of the positive charge on its nucleus4 ,the atomic number could then be set equal to that charge and be made independent of new discoveries of elements4 ,hydrogen1 for instance1 has an atomic number of #a4 ,its nucleus carries a unit positive charge1 #a1 and the hydrogen atom possesses #a electron to balance this4 ,helium1 with an atomic number of #b1 has a nuclear charge of #b and #b electrons1 with a total charge of -#b1 to balance it4 7,the alpha particle released by radioactive atoms is identical with a helium nucleus47 ,the atomic number increases as one goes up the line of atoms4 ,oxygen atoms1 for instance1 have an atomic number of #h and iron atoms have one of #bf4 ,at the upper end1 thorium is #ij and uranium is #ib4 ,each uranium atom has a nucleus bearing a charge of #ib and contains #ib electrons to balance this4 ,once the notion of the atomic number was worked out1 it became possible to tell for certain whether any elements remained as yet undiscovered and1 if so1 where in the list they might be4 ,thus1 when ,moseley first presented scientists with the atomic number it turned out that there were still #g elements that were not discovered4 ,at least elements with atomic numbers of #dc1 #fa1 #gb1 #ge1 #he1 #hg1 and #ia were still not known4 ,by #aide1 all seven had been discovered4 ,it quickly turned out that the atomic number was more fundamental and more characteristic of a particular element than was the atomic weight4 ,illustration3 ,niels ,bohr ,illustration3 ,bohr's study4 ,since ,dalton's time it had been assumed that all the atoms of a particular element were of equal atomic weight and that atoms of two different elements were always of different atomic weight4 ,the first inkling and the first proof that this might not be so came through the study of radioactivity4 ,illustration3 showing ,helium atom1 ,hydrogen atom2 ,nucleus1 ,proton1 ,neutron1 ,electron labelled ,isotopes ,in #aijb ,rutherford and his co-worker ,frederick ,soddy 7#ahgg-aief7 showed that when uranium atoms gave off alpha particles1 a new kind of atom was formed that was not uranium at all4 ,it was this new atom that was eventually found to give off a beta particle1 and then another atom of still another element was formed4 ,this work of ,rutherford and ,soddy began a line of investigation that by #aijg had shown that there was a whole radioactive chain of elements1 each one breaking down to the next in line by giving off either an alpha particle or a beta particle1 until finally a lead atom was formed that was not radioactive4 ,illustration3 ,frederick ,soddy ,there was1 in short1 a 8radioactive series0 beginning with uranium 7atomic number #ib7 and ending with lead 7atomic number #hb74 ,the same was true of thorium 7atomic number #ij71 which began a series that also ended with lead4 ,still a third element1 actinium 7atomic number #hi7 was1 at that time1 the first known member of a series that also ended in lead4 ,the various atoms formed in these three radioactive series were not all different in every way4 ,when the uranium atom gives off an alpha particle1 it forms an atom originally called 8uranium ,x04 ,on close investigation1 it turned out that this uranium ,x had the chemical properties of thorium4 ,uranium ,x1 had1 however1 radioactive properties different from ordinary thorium4 ,uranium ,x broke down so rapidly1 giving off beta particles as it did so1 that half of any given quantity would have broken down in #bd days4 ,another way of saying this 7which was introduced by ,rutherford7 was that the 8half-life0 of uranium ,x1 is #bd days4 ,ordinary thorium1 however1 gives off alpha particles1 not beta particles1 and does so at such a slow rate1 that its half-life is #ad billion years6 ,uranium ,x1 and ordinary thorium were in the same place in the list of elements by chemical standards1 and yet there was clearly something different about the two4 ,here is another case4 ,in #aiac the ,british chemist ,alexander ,fleck 7#ahhi- 7 studied 8radium ,b0 and 8radium ,d01 the names given to two different kinds of atoms in the uranium radioactive series4 ,he also studied 8thorium ,b0 in the thorium radioactive series and 8actinium ,b0 in the actinium radioactive series4 ,all four are chemically the same as ordinary lead2 all four are in the same place in the list of elements4 ,yet each is different from the radioactive standpoint4 ,though all give off beta particles1 radium ,b has a half-life of #bg minutes1 radium ,d one of #ai years1 thorium ,b one of #aa hours1 and actinium ,b one of #cf minutes4 ,in #aiac ,soddy called atoms that were in the same place in the list of elements1 but which had different radioactive properties1 8isotopes01 from ,greek words meaning 8same place04 ,at first1 it seemed that the only difference between isotopes might be in their radioactive properties and that only radioactive atoms were involved4 ,quickly that proved not to be so4 ,it proved that it was possible to have several forms of the same element that were all different even though none of them were radioactive4 ,the uranium series1 the thorium series1 and the actinium series all ended in lead4 ,in each case the lead formed was stable 7not radioactive74 ,were the lead atoms identical in every case8 ,soddy had worked out the way in which atomic weights altered every time an alpha particle or a beta particle was given off by an atom4 ,working through the three radioactive series he decided that the lead atoms had different atomic weights in each case4 ,the uranium series ought to end with lead atoms that had an atomic weight of #bjf4 ,the thorium series ought to end in lead atoms with an atomic weight of #bjh and the actinium series in lead atoms with an atomic weight of #bjg4 ,if this were so1 there would be #c lead isotopes that would differ not in radioactive properties1 but in atomic weight4 ,the isotopes could be referred to as lead-#bjf1 lead-#bjg1 and lead-#bjh4 ,if we use the chemical symbol for lead 7,pb71 we could write the isotopes1 ,pb1 ,pb1 and ,pb4 7,we read the symbol ,pb as lead-#bjf47 ,atomic weight measurements made in #aiad by ,soddy and others supported that theory4 ,all #c lead isotopes had the same atomic number of #hb4 ,the atoms of all #c isotopes had nuclei with an electric charge of #hb and all #c had #hb electrons in the atom to balance that positive nuclear charge4 ,the difference was in the mass of the nucleus only4 ,illustration3 ,isotopes of two elements4 ,atomic number1 #a ,hydrogen-#a3 ,mass number1 #a2 #a ,proton1 #a ,electron ,hydrogen-#b3 ,mass number1 #b2 #a ,proton1 #a ,neutron1 #a ,electron ,atomic number1 #b ,helium-#c3 ,mass number1 #c2 #b ,protons1 #a ,neutron1 #b ,electrons ,helium-#d3 ,mass number1 #d2 #b ,protons1 #b ,neutrons1 #b ,electrons ,but what of ordinary lead that existed in the rocks far removed from any radioactive substances and that had presumably been stable through all the history of earth8 ,its atomic weight was #bjg4b4 ,was the stable lead that had no connection with radioactivity made up of atoms of still another isotope1 one with a fractional atomic weight8 ,or could stable lead be made up of a mixture of isotopes1 each of a different whole-number atomic weight and was the overall atomic weight a fraction only because it was an average8 ,it was at the moment difficult to tell in the case of lead1 but an answer came in connection with another element1 the rare gas neon 7atomic symbol ,ne71 which has an atomic weight of #bj4b4 ,was that fractional atomic weight something that was possessed by all neon atoms without exception or was it the average of some lightweight atoms and some heavyweight ones8 ,it would be a matter of crucial importance if isotopes of neon could be found1 for neon had nothing to do with any of the radioactive series4 ,if neon had isotopes then any element might have them4 ,in #aiab ,thomson was working on neon4 ,he sent a stream of cathode-ray electrons through neon gas4 ,the electrons smashed into the neon atoms and knocked an electron off some of them4 ,that left a neon ion carrying a single positive charge--an ion that could be written ,ne4 ,the neon ions move in the electric field as electrons do1 but in the opposite direction since they have an opposite charge4 ,in the combined presence of a magnet and of an electric field1 the neon ions move in a curved path4 ,if all the neon ions had the same mass1 all would follow the same curve4 ,if some were more massive than others1 the more massive ones would curve less4 ,the neon ions ended on a photographic plate1 which was darkened at the point of landing4 ,there were two regions of darkening1 because there were neon ions of two different masses that curved in two different degrees and ended in two different places4 ,thomson showed1 from the amount of curving1 that there was a neon isotope with an atomic weight of #bj and one with an atomic weight of #bb--,ne and ,ne4 ,what's more1 from the intensity of darkening1 it could be seen that ordinary neon was made up of atoms that were roughly #ij ,ne and #aj ,ne4 ,the overall atomic weight of neon1 #bj4b1 was the average atomic weight of these #b isotopes4 ,thomson's instrument was the first one capable of separating isotopes and such instruments came to be called 8mass spectrometers04 ,the first to use the name was the ,english physicist ,francis ,william ,aston 7#ahgg-aide71 who built the first efficient instrument of this type in #aiai4 ,he used it to study as many elements as he could4 ,he and those who followed him located many isotopes and determined the frequency of their occurrence with considerable precision4 ,it turned out1 for instance1 that neon is actually #ij4i ,ne1 and #h4h ,ne4 ,very small quantities of still a third isotope1 ,ne1 are also present1 making up #j4c4 ,as for ordinary lead in nonradioactive rocks1 it is made up of #bc4f ,pb1 #bb4f ,pb1 and #eb4c ,pb4 ,there is still a fourth isotope1 ,pb1 which makes up the remaining #a4e and which is not the product of any radioactive series at all4 ,the isotopes always have atomic weights that are close to1 but not quite1 whole numbers4 ,any atomic weight of an element that departs appreciably from an integer does so only because it is an average of different isotopes4 ,for instance1 the atomic weight of chlorine 7chemical symbol ,cl7 is #ce4e1 but this is because it is made up of a mixture of #b isotopes4 ,about one quarter of chlorine's atoms are ,cl and about three-quarters are ,cl4 ,illustration3 ,francis ,w4 ,aston ,illustration3 ,mass spectrograph as used by ,thomson and ,aston to measure the atomic weight of neon4 ,to avoid confusion1 the average mass of the isotopes that make up a particular element is still called the atomic weight of that element4 ,the integer closest to the mass of the individual isotope is spoken of as the 8mass number0 of that isotope4 ,thus1 chlorine is made up of isotopes with mass numbers #ce and #cg1 but the atomic weight of chlorine as it is found in nature is #ce4e 7or1 to be more accurate1 #ce4dec74 ,in the same way1 ordinary lead is made up of isotopes with mass numbers #bjd1 #bjf1 #bjg1 and #bjh1 and its atomic weight is #bjg4ai2 neon is made up of isotopes with mass numbers #bj1 #ba1 and #bb1 and its atomic weight is #bj4ahc1 and so on4 ,if the atomic weight of some element happens to be very close to a whole number to begin with1 it may consist of a single kind of atom4 ,for instance1 the gas fluorine 7chemical symbol ,f7 has an atomic weight of nearly #ai1 while that of the metal sodium 7chemical symbol ,na7 is nearly #bc4 ,as it turns out1 all the atoms of fluorine are of the single variety ,f1 while all the atoms of sodium are ,na4 ,sometimes the atomic weight of an element1 as it occurs in nature1 is nearly a whole number and yet it is made up of more than #a isotope4 ,in that case1 one of the isotopes makes up very nearly all of it1 while the others are present in such minor quantities that the average is hardly affected4 ,helium1 for instance 7atomic symbol ,he7 has an atomic weight of just about #d and1 indeed1 almost all the atoms making it up are ,he4 ,however1 #j4jjja of the atoms1 or one out of a million1 are ,he4 ,again1 #ii4f of all the nitrogen atoms 7atomic symbol ,n7 are ,n1 but #j4d are ,n4 ,then1 #ih4i of all carbon atoms 7atomic symbol ,c7 are ,c1 but #a4a are ,c4 ,it is not surprising that the atomic weights of nitrogen and carbon are just about #ad and #ab1 respectively4 ,illustration3 ,harold ,urey ,even hydrogen does not escape4 ,its atomic weight is just about #a and most of its atoms are ,h4 ,the ,american chemist ,harold ,clayton ,urey 7#ahic- 7 detected the existence of a more massive isotope1 ,h4 ,this isotope has almost twice the mass of the lighter one4 ,no other isotopes of a particular atom differ in mass by so large a factor4 ,for that reason ,h and ,h differ in ordinary chemical properties more than isotopes usually do and ,urey therefore gave ,h the special name of 8deuterium0 from a ,greek word meaning 8second04 ,illustration3 ,w4 ,f4 ,giauque ,in #aibi the ,american chemist ,william ,francis ,giauque 7#ahie- 7 found that oxygen was composed of more than #a isotope4 ,its atomic weight had been set arbitrarily at #af4jjjj so it was a relief that #ii4gf of its atoms were ,o4 ,however1 #j4bj were ,o1 and #j4jd were ,o4 ,as you see1 ,o must have a mass number of slightly less than #af4jjjj and it must be the more massive isotopes ,o and ,o that pull the average up to #af4jjjj4 ,disregarding this1 chemists clung to a standard atomic weight of #af4jjj for oxygen as it appeared in nature1 preferring not to concern themselves with the separate isotopes4 ,physicists1 however1 felt uneasy at using an average as standard for they were more interested in working with individual isotopes4 ,they preferred to set ,o at #af4jjjj so that the average atomic weight of oxygen was #af4jjdd and all other atomic weights rose in proportion4 ,atomic weights determined by this system were 8physical atomic weights04 ,finally1 in #aifa1 a compromise was struck4 ,chemists and physicists alike decided to consider the atomic weight of ,c as exactly #ab and to use that as a standard4 ,by this system1 the atomic weight of oxygen became #ae4iiid1 which is only very slightly less than #af4 ,the radioactive elements did not escape this new view either4 ,the atomic weight of uranium 7chemical symbol ,u7 is just about #bch and1 indeed1 most of its atoms are ,u4 ,in #aice1 however1 the ,canadian-,american physicist1 ,arthur ,jeffrey ,dempster 7#ahhf-aiej71 found that #j4g of its atoms were a lighter isotope1 ,u4 ,these differed considerably in radioactive properties4 ,the common uranium isotope1 ,u1 had a half-life of #dejj million years1 while ,u had a half-life of only #gjj million years4 ,furthermore ,u broke down in three stages to actinium4 ,it was ,u1 not actinium itself1 that was the beginning of the actinium radioactive series4 ,as for thorium 7atomic symbol ,th7 with an atomic weight of #bcb1 it did indeed turn out that in the naturally occurring element virtually all the atoms were ,th4 ,,energy ,the ,law of ,conservation of ,energy ,we have now gone as far as we conveniently can in considering the intertwining strands of the atom and of electricity4 ,it is time to turn to the third strand--energy4 ,to physicists the concept of 8work0 is that of exerting a force on a body and making it move through some distance4 ,to lift a weight against the pull of gravity is work4 ,to drive a nail into wood against the friction of its fibers is work4 ,anything capable of performing work is said to possess 8energy0 from ,greek words meaning 8work within04 ,there are various forms of energy4 ,any moving mass possesses energy by virtue of its motion4 ,that is1 a moving hammer will drive a nail into wood1 while the same hammer held motionlessly against the nailhead will not do so4 ,heat is a form of energy1 since it will expand steam that will force wheels into motion that can then do work4 ,electricity1 magnetism1 sound1 and light can be made to perform work and are forms of energy4 ,the forms of energy are so many and so various that scientists were eager to find some rule that covered them all and would therefore serve as a unifying bond4 ,it did not seem impossible that such a rule might exist1 since one had been found in connection with matter that appeared in even greater variety than energy did4 ,all matter1 whatever its form and shape1 possessed mass1 and in the #aggjs1 the ,french chemist ,antoine ,laurent ,lavoisier 7#agdc-agid7 discovered that the quantity of mass was constant4 ,if a system of matter were isolated and made to undergo complicated chemical reactions1 everything about it might change1 but not its mass4 ,a solid might turn into a gas2 a single substance might change into two or three different substances1 but whatever happened1 the total mass at the end was exactly the same 7as nearly as chemists could tell7 as at the beginning4 ,none was either created or destroyed1 however1 the nature of the matter might change4 ,this was called the 8law of conservation of mass04 ,illustration3 ,lavoisier in his laboratory during his studies on respiration4 ,from a sketch made by ,madame ,lavoisier4 ,illustration3 ,antoine ,lavoisier and his wife4 ,naturally1 it would occur to scientists to wonder if a similar law might hold for energy4 ,the answer wasn't easy to get4 ,it wasn't as simple to measure the quantity of energy as it was to measure the quantity of mass4 ,nor was it as simple to pen up a quantity of energy and keep it from escaping or from gaining additional quantity from outside1 as it was in the case of mass4 ,beginning in #ahdj1 however1 the ,english physicist ,james ,prescott ,joule 7#ahah-ahhi7 began a series of experiments in which he made use of every form of energy he could think of4 ,in each case he turned it into heat and allowed the heat to raise the temperature of a given quantity of water4 ,he used the rise in temperature as a measure of the energy4 ,by #ahdg he was convinced that any form of energy could be turned into fixed and predictable amounts of heat2 that a certain amount of work was equivalent to a certain amount of heat4 ,in that same year1 the ,german physicist ,hermann ,ludwig ,ferdinand von ,helmholtz 7#ahba-ahid7 advanced the general notion that a fixed amount of energy in one form was equal to the same amount of energy in any other form4 ,energy might change its form over and over1 but not change its amount4 ,none could either be destroyed or created4 ,this is the 8law of conservation of energy04 ,chemical ,energy ,there is energy in a piece of wood4 ,left quietly to itself1 it seems completely incapable of bringing about any kind of work4 ,set it on fire1 however1 and the wood plus the oxygen in the air will give off heat and light that are clearly forms of energy4 ,the heat could help boil water and run a steam engine4 ,the amount of energy in burning wood could be measured if it were mixed with air and allowed to burn in a closed container that was immersed in a known quantity of water4 ,from the rise in temperature of the water1 the quantity of energy produced could be measured in units called 8calories0 7from a ,latin word for 8heat074 ,the instrument was therefore called a 8calorimeter04 ,in the #ahfjs the ,french chemist ,pierre ,eugne ,marcelin ,berthelot 7#ahbg-aijg7 carried through hundreds of such determinations4 ,his work and similar work by others made it clear that such 8chemical energy0--the energy derived from chemical changes in matter--fit the law of conservation of energy4 ,here's how it looked in the last decades of the #aith century4 ,molecules are composed of combinations of atoms4 ,within the molecules1 the atoms stick together more or less tightly4 ,it takes a certain amount of energy to pull a molecule apart into separate atoms against the resistance of the forces holding them together4 ,if1 after being pulled apart1 the atoms are allowed to come together again1 they give off energy4 ,the amount of energy they give off in coming together is exactly equal to the amount of energy they had to gain before they could separate4 ,this is true of all substances4 ,for instance1 hydrogen gas1 as it is found on earth1 is made up of molecules containing #b hydrogen atoms each 7,h74 ,add a certain amount of energy and you pull the atoms apart2 allow the atoms to come back together into paired molecules1 and the added energy is given back again4 ,the same is true for the oxygen molecule1 which is made up of #b oxygen atoms 7,o7 and of the water molecule 7,h,o74 ,always the amount of energy absorbed in one change is given off in the opposite change4 ,the amount absorbed and the amount given off are always exactly equal4 ,however1 the amount of energy involved differs from molecule to molecule4 ,it is quite hard to pull hydrogen molecules apart1 and it is even harder to pull oxygen molecules apart4 ,you have to supply about #ab more energy to pull an oxygen molecule apart than to pull a hydrogen molecule apart4 ,naturally1 if you let #b oxygen atoms come together to form an oxygen molecule1 you get back #ab more energy than if you allow #b hydrogen atoms to come together to form a hydrogen molecule4 ,it takes a considerably larger amount of energy to pull apart a water molecule into separate atoms than to pull apart either hydrogen or oxygen molecules4 ,naturally1 that greater energy is also returned once the hydrogen and oxygen atoms are allowed to come back together into water molecules4 ,next1 imagine pulling apart hydrogen and oxygen molecules into hydrogen and oxygen atoms and then having those atoms come together to form water molecules4 ,a certain amount of energy is put into the system to break up the hydrogen and oxygen molecules1 but then a much greater amount of energy is given off when the water molecules form4 ,it is for that reason that a great deal of energy 7mostly in the form of heat7 is given off if a jet of hydrogen gas and a jet of oxygen gas are allowed to mix in such a way as to form water4 ,just mixing the hydrogen and oxygen isn't enough4 ,the molecules of hydrogen and oxygen must be separated and that takes a little energy4 ,the energy in a match flame is enough to raise the temperature of the mixture and to make the hydrogen and oxygen molecules move about more rapidly and more energetically4 ,this increases the chance that some molecules will be broken up into separate atoms 7though the actual process is rather complicated74 ,an oxygen atom might then strike a hydrogen molecule to form water 7,o ,h ,h,o7 and more energy is given off than was absorbed from the match flame4 ,the temperature goes up still higher so that further breakup among the oxygen and hydrogen molecules is encouraged4 ,illustration3 ,the formation of a sodium chloride molecule4 ,this happens over and over again so that in very little time1 the temperature is very high and the hydrogen and oxygen are combining to form water at an enormous rate4 ,if a great deal of hydrogen and oxygen are well-mixed to begin with1 the rate of reaction is so great that an explosion occurs4 ,such a situation1 in which each reacting bit of the system adds energy to the system by its reaction and brings about more reactions like itself1 is called a 8chain reaction04 ,thus1 a match flame put to one corner of a large sheet of paper will set that corner burning4 ,the heat of the burning will ignite a neighboring portion of the sheet and so on till the entire sheet is burned4 ,for that matter a single smoldering cigarette end can serve to burn down an entire forest in a vastly destructive chain reaction4 ,electrons and ,energy ,the discovery of the structure of the atom sharpened the understanding of chemical energy4 ,in #aijd the ,german chemist ,richard ,abegg 7#ahfi-aiaj7 first suggested that atoms were held together through the transfer of electrons from one atom to another4 ,to see how this worked1 one began by noting that electrons in an atom existed in a series of shells4 ,the innermost shell could hold only #b electrons1 the next #h1 the next #ah and so on4 ,it turned out that some electron arrangements were more stable than others4 ,if only the innermost shell contained electrons and it were filled with the #b electrons that were all it could hold1 then that was a stable arrangement4 ,if an atom contained electrons in more than one shell and the outermost shell that held electrons held #h1 that was a stable arrangement1 too4 ,thus1 the helium atom has #b electrons only1 filling the innermost shell1 and that is so stable an arrangement that helium undergoes no chemical reactions at all4 ,the neon atom has #aj electrons--#b in the innermost shell1 and #h in the next--and it does not react4 ,the argon atom has #ah electrons--#b1 #h1 and #h--;and it too is very stable4 ,but what if an atom did not have its electron shell so neatly filled4 ,the sodium atom has #aa electrons--#b1 #h1 and #a--while the fluorine atom has #i electrons--#b and #g4 ,if the sodium atom passed one of its electrons to a fluorine atom1 both would have the stable configuration of neon--#b and #h4 ,this1 therefore1 ought to have a great tendency to happen4 ,if it did happen1 though1 the sodium atom1 minus #a electron1 would have a unit positive charge and would be ,na1 a positively charged ion4 ,fluorine with #a electron in excess would become ,f1 a negatively charged ion4 ,the #b ions1 with opposite charges1 would cling together1 since opposite charges attract1 and thus the molecule of sodium fluoride 7,na,f7 would be formed4 ,in #aiaf the ,american chemist ,gilbert ,newton ,lewis 7#ahge-aidf7 carried this notion farther4 ,atoms could cling together not only as a result of the outright transfer of #a or more electrons1 but through sharing pairs of electrons4 ,this sharing could only take place if the atoms remained close neighbors1 and it would take energy to pull them apart and break up the shared pool1 just as it would take energy to pull #b ions apart against the attraction of opposite charges4 ,in this way the vague notions of atoms clinging together in molecules and being forced apart gave way to a much more precise picture of electrons being transferred or shared4 ,the electron shifts could be dealt with mathematically by a system that came to be called 8quantum mechanics0 and chemistry was thus made a more exact science than it had ever been before4 ,the ,energy of the ,sun ,the most serious problem raised by the law of conservation of energy involved the sun4 ,until #ahdg1 scientists did not question sunlight4 ,the sun radiated vast quantities of energy but that apparently was its nature and was no more to be puzzled over than the fact that the earth rotated on its axis4 ,once ,helmholtz had stated that energy could neither be created nor destroyed1 however1 he was bound to ask where the sun's energy came from4 ,it had1 to man's best knowledge1 been radiating heat and light1 with no perceptible change1 throughout the history of civilization and1 from what biologists and geologists could deduce1 for countless ages earlier4 ,where1 then1 did that energy come from8 ,the sun gave the appearance of being a huge globe of fire4 ,could it actually be that--a large heap of burning fuel1 turning chemical energy into heat and light8 ,the sun's mass was known and its rate of energy production was known4 ,suppose the sun's mass were a mixture of hydrogen and oxygen and it were burning at a rate sufficient to produce the energy at the rate it was giving it off4 ,if that were so1 all the hydrogen and oxygen in its mass would be consumed in #aejj years4 ,no chemical reaction in the sun could account for its having given us heat and light since the days of the pyramids1 let alone since the days of the dinosaurs4 ,was there some source of energy greater than chemical energy8 ,what about the energy of motion8 ,helmholtz suggested that meteors might be falling into the sun at a steady rate4 ,the energy of their collisions might then be converted into heat and light and this could keep the sun shining for as long as the supply of meteors held out--even millions of years4 ,this1 however1 would mean that the sun's mass would be increasing steadily1 and so would the force of its gravitational pull4 ,with the sun's gravitational field increasing steadily1 the length of earth's year would be decreasing at a measurable rate--but it wasn't4 ,in #ahed ,helmholtz came up with something better4 ,he suggested that the sun was contracting4 ,its outermost layers were falling inward1 and the energy of this fall was converted into heat and light4 ,what's more1 this energy would be obtained without any change in the mass of the sun whatever4 ,helmholtz calculated that the sun's contraction over the #fjjj years of recorded history would have reduced its diameter only #efj miles--a change that would not have been noticeable to the unaided eye4 ,since the development of the telescope1 two and a half centuries earlier1 the decrease in diameter would have been only #bc miles and that was not measurable by the best techniques of ,helmholtz's day4 ,working backward1 however1 it seemed that #be million years ago1 the sun must have been so large as to fill the earth's orbit4 ,clearly the earth could not then have existed4 ,in that case1 the maximum age of the earth was only #be million years4 ,geologists and biologists found themselves disturbed by this4 ,the slow changes in the earth's crust and in the evolution of life made it seem very likely that the earth must have been in existence--with the sun delivering heat and light very much in the present fashion--for many hundreds of millions of years4 ,yet there seemed absolutely no other way of accounting for the sun's energy supply4 ,either the law of conservation of energy was wrong 7which seemed unlikely71 or the painfully collected evidence of geologists and biologists was wrong 7which seemed unlikely71--or there was some source of energy greater than any known in the #aith century1 whose existence had somehow escaped mankind 7which also seemed unlikely74 ,yet one of those unlikely alternatives would have to be true4 ,and then in #ahif came the discovery of radioactivity4 ,the ,energy of ,radioactivity ,it eventually became clear that radioactivity involved the giving off of energy4 ,uranium emitted gamma rays that we now know to be a hundred thousand times as energetic as ordinary light rays4 ,what's more1 alpha particles were being emitted at velocities of perhaps #cj1jjj kilometers per second1 while the lighter beta particles were being shot off at velocities of up to #bej1jjj kilometers per second 7about #j4h times the velocity of light74 ,at first1 the total energy given off by radioactive substances seemed so small that there was no use worrying about it4 ,the amount of energy liberated by a gram of uranium in #a second of radioactivity was an insignificant fraction of the energy released by a burning candle4 ,in a few years1 however1 something became apparent4 ,a lump of uranium might give off very little energy in a second1 but it kept on for second after second1 day after day1 month after month1 and year after year with no perceptible decrease4 ,the energy released by the uranium over a very long time grew to be enormous4 ,it eventually turned out that while the rate at which uranium delivered energy did decline1 it did so with such unbelievable slowness that it took #d4e billion years 767 for that rate to decrease to half what it was to begin with4 ,if all the energy delivered by a gram of uranium in the course of its radioactivity over many billions of years was totalled1 it was enormously greater than the energy produced by the burning of a candle with a mass equal to that of uranium4 ,let's put it another way4 ,we might think of a single uranium atom breaking down and shooting off an alpha particle4 ,we might also think of a single carbon atom combining with #b oxygen atoms to form carbon dioxide4 ,the uranium atom would give off #b1jjj1jjj times as much energy in breaking down1 as the carbon atom would in combining4 ,the energy of radioactivity is millions of times as intense as the energy released by chemical reactions4 ,the reason mankind had remained unaware of radioactivity and very aware of chemical reactions was1 first1 that the most common radioactive processes are so slow that their great energies were stretched over such enormous blocks of time as to be insignificant on a per second basis4 ,secondly1 chemical reactions are easily controlled by changing quantities1 concentrations1 temperatures1 pressures1 states of mixtures1 and so on1 and this makes them easy to take note of and to study4 ,the rate of radioactive changes1 however1 could not apparently be altered4 ,the early investigators quickly found that the breakdown of uranium-#bch1 for instance1 could not be hastened by heat1 pressure1 changes in chemical combination1 or1 indeed1 anything else they could think of4 ,it remained incredibly slow4 ,but despite all this1 radioactivity had at last been discovered and the intensity of its energies was recognized and pointed out in #aijb by ,marie ,curie and her husband ,pierre ,curie 7#ahei-aijf74 ,where1 then1 did the energy come from8 ,could it come from the outside8 ,could the radioactive atoms somehow collect energy from their surroundings1 concentrate it several million-fold1 and then let it out all at once8 ,to concentrate energy in this fashion would violate something called 8the second law of thermodynamics04 ,this was first proposed in #ahej by the ,german physicist ,rudolf ,julius ,emmanuel ,clausius 7#ahbb-ahhh7 and had proved so useful that physicists did not like to abandon it unless they absolutely had to4 ,another possibility was that radioactive atoms were creating energy out of nothing4 ,this1 of course1 violated the law of conservation of energy 7also called 8the first law of thermodynamics07 and physicists preferred not to do that either4 ,the only thing that seemed to remain was to suppose that somewhere within the atom was a source of energy that had never made itself evident to humanity until the discovery of radioactivity4 ,becquerel was one of the first to suggest this4 ,it might have seemed at first that only radioactive elements had this supply of energy somewhere within the atom1 but in #aijc ,rutherford suggested that all atoms had a vast energy supply hidden within themselves4 ,the supply in uranium and thorium leaked slightly1 so to speak1 and that was all that made them different4 ,illustration3 ,the room in which the ,curies discovered radium4 ,pierre ,curie's writing is on the blackboard4 ,but if a vast supply of energy existed in atoms1 it was possible that the solution to the puzzle of the sun's energy might rest there4 ,as early as #ahii the ,american geologist ,thomas ,chrowder ,chamberlin 7#ahdc-aibh7 was already speculating about a possible connection between radioactivity and the sun's energy4 ,if it were some variety of this newly discovered source of energy 7not necessarily ordinary radioactivity1 of course7 that powered the sun--millions of times as intense as chemical energy--then the sun might be pouring out energy for hundreds of millions of years without perceptible physical change--just as uranium would show scarcely any change even in so mighty a time span4 ,the sun would not have to be contracting2 it would not have had to fill the earth's orbit #be1jjj1jjj years ago4 ,this was all exciting1 but in #aijj the structure of the atom had not yet been worked out and this new energy was just a vague supposition4 ,no one had any idea of what it actually might be or where in the atom it might be located4 ,it could only be spoken of as existing 8within the atom0 and was therefore called 8atomic energy04 ,through long habit1 it is still called that much of the time4 ,and yet 8atomic energy0 is not a good name4 ,in the first couple of decades of the #bjth century1 it became apparent that ordinary chemical energy involved electron shifts and those electrons were certainly components of atoms4 ,this meant that a wood fire was a kind of atomic energy4 ,the electrons1 however1 existed only in the outer regions of the atom4 ,once ,rutherford worked out the theory of the nuclear atom1 it became apparent that the energy involved in radioactivity and in solar radiation had to involve components of the atom that were more massive and more energetic than the light electrons4 ,the energy had to come1 somehow1 from the atomic nucleus4 ,what is involved then in radioactivity and in the sun is 8nuclear energy04 ,that is the proper name for it and in the next section we will consider the subsequent history of the nuclear energy that broke upon the startled consciousness of scientists as the #bjth century opened and which1 less than half a century later1 was to face mankind with untold consequences for good and for evil4 ,,footnotes #a8,mass0 is the correct term1 but 8weight01 which is a somewhat different thing1 is so commonly used instead that in this book ,i won't try to make any distinction4 ,,quotation ,,credit ,inside front cover ,copyright by ,abelard-,shuman1 ,ltd41 ,new ,york4 ,reprinted by permission from ,inside the ,atom1 ,isaac ,asimov1 #aiff4 ,,photo ,,credits ,cover ,the ,metropolitan ,museum of ,art ,page facing inside ,the 8,horsehead0 ,nebula in ,orion4 ,hale cover ,observatories4 ,author's ,photo ,jay ,k4 ,klein ,contents page ,lick ,observatory page #d ,page #g ,new ,york ,public ,library #i ,from ,discovery of the ,elements1 ,mary ,e4 ,weeks1 ,chemical ,education ,publishing ,company1 #aifh4 #ab ,library of ,congress #ae ,sir ,george ,thomson #ah ,burndy ,library #ai ,new ,york ,public ,library #ba ,copyright #aife by ,barbara ,lovett ,cline1 reprinted from her volume ,the ,questioners3 ,physicists and the ,quantum ,theory by permission of ,thomas ,y4 ,crowell ,company1 ,inc41 ,new ,york4 #bb #bc ,curie ,foundation1 ,institute of ,radium #bf ,academic ,press1 ,inc4 #bi ,van ,nostrand ,reinhold ,company #ca ,top1 ,nobel ,institute2 bottom1 from ,discovery of the ,elements1 ,mary ,e4 ,weeks1 ,chemical ,education ,publishing ,company1 #aifh4 #cb ,from ,discovery of the ,elements1 ,mary ,e4 ,weeks1 ,chemical ,education ,publishing ,company1 #aifh4 #cd ,top1 ,nobel ,institute2 bottom1 ,niels ,bohr ,institute4 #cf1 #db1 #dd1 #de ,nobel ,institute #dh ,academic ,press1 ,inc4 #di ,from ,discovery of the ,elements1 ,mary ,e4 ,weeks1 ,chemical ,education ,publishing ,company1 #aifh4 #fj ,curie ,foundation1 ,institute of ,radium ,u4,s4 ,,government ,,printing ,,office3 #aige--fdj--bhe_/ac ,a word about ,,erda4444 ,the mission of the ,u4 ,s4 ,energy ,research ,development ,administration 7,,erda7 is to develop all energy sources1 to make the ,nation basically self-sufficient in energy1 and to protect public health and welfare and the environment4 ,,erda programs are divided into six major categories3 ,,conservation ,,of ,,energy--,more efficient use of existing energy sources1 development of alternate fuels and engines for automobiles to reduce dependence on petroleum1 and elimination of wasteful habits of energy consumption4 ,,fossil ,,energy--,expansion of coal production and the development of technologies for converting coal to synthetic gas and liquid fuels1 improvement of oil drilling methods and of techniques for converting shale deposits to usable oil4 ,,solar1 ,,geothermal1 ,,and ,,advanced ,,energy ,,systems--,research on solar energy to heat1 cool1 and eventually electrify buildings1 on conversion of underground heat sources to gas and electricity1 and on fusion reactors for the generation of electricity4 ,,environment ,,and ,,safety--,investigation of health1 safety1 and environmental effects of the development of energy technologies1 and research on management of wastes from energy production4 ,,nuclear ,,energy--,expanding medical1 industrial and research applications and upgrading reactor technologies for the generation of electricity1 particularly using the breeder concept4 ,,national ,,security--,production and administration of nuclear materials serving both civilian and military needs4 ,,erda programs are carried out by contract and cooperation with industry1 university communities1 and other government agencies4 ,for more information1 write to ,,userda--,technical ,information ,center1 ,p4 ,o4 ,box #fb1 ,oak ,ridge1 ,tennessee #cghcj4 ,illustration3 ,,energy ,,research ,,development ,,administration ,,usa ,united ,states ,energy ,research and ,development ,administration ,office of ,public ,affairs ,washington1 ,d4,c4 #bjede ,transcriber's ,notes --,retained publication information from the printed edition3 this e,book is public-domain in the country of publication4 --,in the text version only1 underlined or italicized text is delimited by underscores4 --,where possible1 ,,utf superscript and subscript numbers are used2 some e-reader fonts may not support these characters4 --,in the text versions only1 other superscript text is preceded by caret and delimited by brackets4 --,in the text versions only1 other subscripted text is preceded by underscore and delimited by brackets4