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y space； with a very dense nucleus at the center。
this was a most gratifying discovery； but it presented one immediate problem。 by all the lawsof conventional physics； atoms shouldn鈥檛 therefore exist。
let us pause for a moment and consider the structure of the atom as we know it now。 everyatom is made from three kinds of elementary particles： protons； which have a positiveelectrical charge； electrons； which have a negative electrical charge； and neutrons； which haveno charge。 protons and neutrons are packed into the nucleus； while electrons spin aroundoutside。 the number of protons is what gives an atom its chemical identity。 an atom with oneproton is an atom of hydrogen； one with two protons is helium； with three protons is lithium；and so on up the scale。 each time you add a proton you get a new element。 （because thenumber of protons in an atom is always balanced by an equal number of electrons； you willsometimes see it written that it is the number of electrons that defines an element； it es tothe same thing。 the way it was explained to me is that protons give an atom its identity；electrons its personality。）neutrons don鈥檛 influence an atom鈥檚 identity； but they do add to its mass。 the number ofneutrons is generally about the same as the number of protons； but they can vary up and downslightly。 add a neutron or two and you get an isotope。 the terms you hear in reference todating techniques in archeology refer to isotopes鈥攃arbon…14； for instance； which is an atomof carbon with six protons and eight neutrons （the fourteen being the sum of the two）。
neutrons and protons occupy the atom鈥檚 nucleus。 the nucleus of an atom is tiny鈥攐nly onemillionth of a billionth of the full volume of the atom鈥攂ut fantastically dense； since itcontains virtually all the atom鈥檚 mass。 as cropper has put it； if an atom were expanded to thesize of a cathedral； the nucleus would be only about the size of a fly鈥攂ut a fly manythousands of times heavier than the cathedral。 it was this spaciousness鈥攖his resounding；unexpected roominess鈥攖hat had rutherford scratching his head in 1910。
it is still a fairly astounding notion to consider that atoms are mostly empty space； and thatthe solidity we experience all around us is an illusion。 when two objects e together in the2geiger would also later bee a loyal nazi； unhesitatingly betraying jewish colleagues； including many whohad helped him。
real world鈥攂illiard balls are most often used for illustration鈥攖hey don鈥檛 actually strike eachother。 鈥渞ather；鈥潯s timothy ferris explains； 鈥渢he negatively charged fields of the two ballsrepel each other 。 。 。 were it not for their electrical charges they could； like galaxies； pass rightthrough each other unscathed。鈥潯hen you sit in a chair； you are not actually sitting there； butlevitating above it at a height of one angstrom （a hundred millionth of a centimeter）； yourelectrons and its electrons implacably opposed to any closer intimacy。
the picture that nearly everybody has in mind of an atom is of an electron or two flyingaround a nucleus； like planets orbiting a sun。 this image was created in 1904； based on littlemore than clever guesswork； by a japanese physicist named hantaro nagaoka。 it ispletely wrong； but durable just the same。 as isaac asimov liked to note； it inspiredgenerations of science fiction writers to create stories of worlds within worlds； in which atomsbee tiny inhabited solar systems or our solar system turns out to be merely a mote in somemuch larger scheme。 even now cern； the european organization for nuclear research； usesnagaoka鈥檚 image as a logo on its website。 in fact； as physicists were soon to realize； electronsare not like orbiting planets at all； but more like the blades of a spinning fan； managing to fillevery bit of space in their orbits simultaneously （but with the crucial difference that the bladesof a fan only seem to be everywhere at once； electrons are ）。
needless to say； very little of this was understood in 1910 or for many years afterward。
rutherford鈥檚 finding presented some large and immediate problems； not least that no electronshould be able to orbit a nucleus without crashing。 conventional electrodynamic theorydemanded that a flying electron should very quickly run out of energy鈥攊n only an instant orso鈥攁nd spiral into the nucleus； with disastrous consequences for both。 there was also theproblem of how protons with their positive charges could bundle together inside the nucleuswithout blowing themselves and the rest of the atom apart。 clearly whatever was going ondown there in the world of the very small was not governed by the laws that applied in themacro world where our expectations reside。
as physicists began to delve into this subatomic realm； they realized that it wasn鈥檛 merelydifferent from anything we knew； but different from anything ever imagined。 鈥渂ecauseatomic behavior is so unlike ordinary experience；鈥潯ichard feynman once observed； 鈥渋t isvery difficult to get used to and it appears peculiar and mysterious to everyone； both to thenovice and to the experienced physicist。鈥潯hen feynman made that ment； physicists hadhad half a century to adjust to the strangeness of atomic behavior。 so think how it must havefelt to rutherford and his colleagues in the early 1910s when it was all brand new。
one of the people working with rutherford was a mild and affable young dane namedniels bohr。 in 1913； while puzzling over the structure of the atom； bohr had an idea soexciting that he postponed his honeymoon to write what became a landmark paper。 becausephysicists couldn鈥檛 see anything so small as an atom； they had to try to work out its structurefrom how it behaved when they did things to it； as rutherford had done by firing alphaparticles at foil。 sometimes； not surprisingly； the results of these experiments were puzzling。
one puzzle that had been around for a long time had to do with spectrum readings of thewavelengths of hydrogen。 these produced patterns showing that hydrogen atoms emittedenergy at certain wavelengths but not others。 it was rather as if someone under surveillancekept turning up at particular locations but was never observed traveling between them。 no onecould understand why this should be。
it was while puzzling over this problem that bohr was struck by a solution and dashed offhis famous paper。 called 鈥渙n the constitutions of atoms and molecules；鈥潯he paper explainedhow electrons could keep from falling into the nucleus by suggesting that they could occupyonly certain well…defined orbits。 according to the new theory； an electron moving betweenorbits would disappear from one and reappear instantaneously in another without visiting thespace between。 this idea鈥攖he famous 鈥渜uantum leap鈥濃�攊s of course utterly strange； but itwas too good not to be true。 it not only kept electrons from spiraling catastrophically into thenucleus； it also explained hydrogen鈥檚 bewildering wavelengths。 the electrons only appearedin certain orbits because they only existed in certain orbits。 it was a dazzling insight； and itwon bohr the 1922 nobel prize in physics； the year after einstein received his。
meanwhile the tireless rutherford； now back at cambridge as j。 j。 thomson鈥檚 successor ashead of the cavendish laboratory； came up with a model that explained why the nuclei didn鈥檛blow up。 he saw that they must be offset by some type of neutralizing particles； which hecalled neutrons。 the idea was simple and appealing； but not easy to prove。 rutherford鈥檚associate； james chadwick； devoted eleven intensive years to hunting for neutrons beforefinally succeeding in 1932。 he； too； was awarded with a nobel prize in physics； in 1935。 asboorse and his colleagues point out in their history of the subject； the delay in discovery wasprobably a very good thing as mastery of the neutron was essential to the development of theatomic bomb。 （because neutrons have no charge； they aren鈥檛 repelled by the electrical fields atthe heart of an atom and thus could be fired like tiny torpedoes into an atomic nucleus； settingoff the destructive process known as fission。） had the neutron been isolated in the 1920s； theynote； it is 鈥渧ery likely the atomic bomb would have be