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en counting alpha particle scintillations； as they wereknown—the sort of work that would normally have been farmed out。 he was one of the firstto see—possibly the very first—that the power inherent in the atom could； if harnessed； makebombs powerful enough to “make this old world vanish in smoke。”
physically he was big and booming； with a voice that made the timid shrink。 once whentold that rutherford was about to make a radio broadcast across the atlantic； a colleague drilyasked： “why use radio？” he also had a huge amount of good…natured confidence。 whensomeone remarked to him that he seemed always to be at the crest of a wave； he responded；“well； after all； i made the wave； didn’t i？” c。 p。 snow recalled how once in a cambridgetailor’s he overheard rutherford remark： “every day i grow in girth。 and in mentality。”
but both girth and fame were far ahead of him in 1895 when he fetched up at thecavendish。
1it was a singularly eventful period in science。 in the year of his arrival incambridge； wilhelm roentgen discovered x rays at the university of würzburg in germany；and the next year henri becquerel discovered radioactivity。 and the cavendish itself wasabout to embark on a long period of greatness。 in 1897； j。 j。 thomson and colleagues woulddiscover the electron there； in 1911 c。 t。 r。 wilson would produce the first particle detectorthere （as we shall see）； and in 1932 james chadwick would discover the neutron there。
further still in the future； james watson and francis crick would discover the structure ofdna at the cavendish in 1953。
in the beginning rutherford worked on radio waves； and with some distinction—hemanaged to transmit a crisp signal more than a mile； a very reasonable achievement for thetime—but gave it up when he was persuaded by a senior colleague that radio had little future。
on the whole； however； rutherford didn’t thrive at the cavendish。 after three years there；feeling he was going nowhere； he took a post at mcgill university in montreal； and there hebegan his long and steady rise to greatness。 by the time he received his nobel prize （for“investigations into the disintegration of the elements； and the chemistry of radioactivesubstances；” according to the official citation） he had moved on to manchester university；and it was there； in fact； that he would do his most important work in determining thestructure and nature of the atom。
1the name es from the same cavendishes who producec henry。 this one was william cavendish； seventhduke of devonshire； who was a gifted mathematician and steel baron in victoriar england。 in 1870； he gave theuniversity ￡6；300 to build an experimental lab。
by the early twentieth century it was known that atoms were made of parts—thomson’sdiscovery of the electron had established that—but it wasn’t known how many parts therewere or how they fit together or what shape they took。 some physicists thought that atomsmight be cube shaped； because cubes can be packed together so neatly without any wastedspace。 the more general view； however； was that an atom was more like a currant bun or aplum pudding： a dense； solid object that carried a positive charge but that was studded withnegatively charged electrons； like the currants in a currant bun。
in 1910； rutherford （assisted by his student hans geiger； who would later invent theradiation detector that bears his name） fired ionized helium atoms； or alpha particles； at asheet of gold foil。
2to rutherford’s astonishment； some of the particles bounced back。 it wasas if； he said； he had fired a fifteen…inch shell at a sheet of paper and it rebounded into his lap。
this was just not supposed to happen。 after considerable reflection he realized there could beonly one possible explanation： the particles that bounced back were striking something smalland dense at the heart of the atom； while the other particles sailed through unimpeded。 anatom； rutherford realized； was mostly empty 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’t 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’t influence an atom’s 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—carbon…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’s nucleus。 the nucleus of an atom is tiny—only onemillionth of a billionth of the full volume of the atom—but fantastically dense； since itcontains virtually all the atom’s 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—but a fly manythousands of times heavier than the cathedral。 it was this spaciousness—this resounding；unexpected roominess—that 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