Uncle Tungsten: Memories of a Chemical Boyhood (2001) (33 page)

The alpha particles emitted by radioactive decay (they were later shown to be helium nuclei) were positively charged and relatively massive – thousands of times more massive than beta particles or electrons – and they traveled in undeviating straight lines, passing straight through matter, ignoring it, without any scattering or deflection (although they might lose some of their velocity in so doing). This, at least, appeared to be the case, though in 1906 Rutherford observed that there might be, very occasionally, small deflections. Others ignored this, but to Rutherford these observations were fraught with possible significance. Would not alpha particles be ideal projectiles, projectiles of atomic proportions, with which to bombard other atoms and sound out their structure? He asked his young assistant Hans Geiger and a student, Ernest Marsden, to set up a scintillation experiment using screens of thin metal foils, so that one could keep count of every alpha particle that bombarded these. Firing alpha particles at a piece of gold foil, they found that roughly one in eight thousand particles showed a massive deflection – of more than 90 degrees, and sometimes even 180 degrees. Rutherford was later to say, ‘It was quite the most incredible event that ever happened to me in my life. It was almost as incredible as if you fired a fifteen-inch shell at a piece of tissue paper and it came back and hit you.’

Rutherford pondered these curious results for almost a year, and then, one day, as Geiger recorded, he ‘came into my room, obviously in the best of moods, and told me that now he knew what the atom looked like and what the strange scatterings signified.’

Atoms, Rutherford had realized, could not be a homogenous jelly of positivity stuck with electrons like raisins (as J.J. Thomson had suggested, in his ‘plum pudding’ model of the atom), for then the alpha particles would always go through them. Given the great energy and charge of these alpha particles, one had to assume that they had been deflected, on occasion, by something even more positively charged than themselves. Yet this happened only once in eight thousand times. The other 7,999 particles might whiz through, undeflected, as if most of the gold atoms consisted of empty space; but the eight-thousandth was stopped, flung back in its tracks, like a tennis ball hitting a globe of solid tungsten. The mass of the gold atom, Rutherford inferred, had to be concentrated at the center, in a minute space, not easy to hit – as a nucleus of almost inconceivable density. The atom, he proposed, must consist overwhelmingly of empty space, with a dense, positively charged nucleus only a hundred-thousandth its diameter, and a relatively few, negatively charged electrons in orbit about this nucleus – a miniature solar system, in effect.

Rutherford’s experiments, his nuclear model of the atom, provided a structural basis for the enormous differences between radioactive and chemical processes, the millionfold differences of energy involved (Soddy would dramatize this, in his popular lectures, by holding a one-pound jar of uranium oxide aloft in one hand – this, he would say, had the energy of a hundred and sixty tons of coal).

Chemical change or ionization involved the addition or removal of an electron or two, and this required only a modest energy of two or three electron-volts, such as could be produced easily – by a chemical reaction, by heat, by light, or by a simple 3-volt battery. But radioactive processes involved the nuclei of atoms, and since these were held together by far greater forces, their disintegration could release energies of far greater magnitude – some millions of electron-volts.

Soddy coined the term
atomic energy
soon after the turn of the nineteenth century, ten years or more before the nucleus was discovered. No one had known, or been able to make a remotely plausible guess, as to how the sun and stars could radiate so much energy, and continue to do so for millions of years. Chemical energy would be ludicrously inadequate – a sun made of coal would burn itself out in ten thousand years. Could radioactivity, atomic energy, provide the answer?

Supposing [wrote Soddy]…our sun…were made of pure radium…there would be no difficulty in accounting for its out-pourings of energy.

Could transmutation, which occurs naturally in radioactive substances, be produced artificially, Soddy wondered.«66» He was moved by this thought to rapturous, millennial, and almost mystical heights:

Radium has taught us that there is no limit to the amount of energy in the world…A race which could transmute matter would have little need to earn its bread by the sweat of its brow…Such a race could transform a desert continent, thaw the frozen poles, and make the whole world one smiling Garden of Eden…An entirely new prospect has been opened up. Man’s inheritance has increased, his aspirations have been uplifted, and his destiny has been ennobled to an extent beyond our present power to foretell…One day he will attain the power to regulate for his own purposes the primary fountains of energy which Nature now so jealously conserves for the future.

I read Soddy’s book
The Interpretation of Radium
in the last year of the war, and I was enraptured by his vision of endless energy, endless light. Soddy’s heady words gave me a sense of the intoxication, the sense of power and redemption, that had attended the discovery of radium and radioactivity at the start of the century.

But side by side with this, Soddy voiced the dark possibilities, too. These indeed had been in his mind almost from the start, and, as early as 1903, he had spoken of the earth as ‘a storehouse stuffed with explosives, inconceivably more powerful than any we know of.’ This note was frequently sounded in
The Interpretation of Radium
, and it was Soddy’s powerful vision that inspired H.G. Wells to go back to his early science-fiction style and publish, in 1914,
The World Set Free
(Wells actually dedicated his book to
The Interpretation of Radium
). Here Wells envisaged a new radioactive element called Carolinum, whose release of energy was almost like a chain reaction:«67»

Always before in the development of warfare the shells and rockets fired had been but momentarily explosive, they had gone off in an instant once and for all…but Carolinum…once its degenerative process had been induced, continued a furious radiation of energy and nothing could arrest it.

I thought of Soddy’s prophesies, and Wells’s, in August of 1945, when we heard the news of Hiroshima. My feelings about the atomic bomb were strangely mixed. Our war, after all, was over, V-E Day was past; unlike the Americans, we had not suffered Pearl Harbor, or the terrible struggles in Guam and Saipan; we had not been in direct combat with the Japanese. The atomic bombings seemed, in some ways, like a terrible postscript to the war, a hideous demonstration that perhaps did not need to be made.

And yet I also had, as many did, a sense of jubilation at the scientific achievement of splitting the atom, and I was enthralled by the Smyth Report, which came out in August of 1945 and gave a full description of the making of the bomb. The full horror of the bomb did not hit me until the following summer, when John Hersey’s ‘Hiroshima’ was published in a special one-article edition of
The New Yorker
(Einstein, it was said, bought a thousand copies of this issue) and broadcast soon after by the BBC on the Third Programme. Up to this point, chemistry and physics had been for me a source of pure delight and wonder, and I was insufficiently conscious, perhaps, of their negative powers. The atomic bombs shook me, as they did everybody. Atomic or nuclear physics, one felt, could never again move with the same innocence and lightheartedness as it had in the days of Rutherford and the Curies.

Brilliant Light

H
ow many elements would God need to build a universe? Fifty-odd elements were known by 1815; and, if Dalton was right, this meant fifty different sorts of atom. But surely God would not need fifty different building blocks for His universe – surely He would have designed it more economically than this. William Prout, a chemically minded physician in London, observing that atomic weights were close to whole numbers and therefore multiples of the atomic weight of hydrogen, speculated that hydrogen was in fact the primordial element, and that all other elements had been built from it. Thus God needed to create only one sort of atom, and all the others, by a natural ‘condensation,’ could be generated from this.

Unfortunately, some elements turned out to have fractional atomic weights. One could round off a weight that was slightly less or slightly more than a whole number (as Dalton did), but what could one do with chlorine, for example, with its atomic weight of 35.5? This made Prout’s hypothesis difficult to maintain, and further difficulties emerged when Mendeleev made the periodic table. It was clear, for example, that tellurium came, in chemical terms, before iodine, but its atomic weight, instead of being less, was greater. These were grave difficulties, and yet throughout the nineteenth century Prout’s hypothesis never really died – it was so beautiful, so simple, many chemists and physicists felt, that it must contain an essential truth.

Was there perhaps some atomic property that was more integral, more fundamental than atomic weight? This was not a question that could be addressed until one had a way of ‘sounding’ the atom, sounding, in particular, its central portion, the nucleus. In 1913, a century after Prout, Harry Moseley, a brilliant young physicist working with Rutherford, set about exploring atoms with the just-developed technique of X-ray spectroscopy. His experimental setup was charming and boyish: using a little train, each car carrying a different element, moving inside a yard-long vacuum tube, Moseley bombarded each element with cathode rays, causing them to emit characteristic X-rays. When he came to plot the square roots of the frequencies against the atomic number of the elements, he got a straight line; and plotting it another way, he could show that the increase in frequency showed sharp, discrete steps or jumps as he passed from one element to the next. This had to reflect a fundamental atomic property, Moseley believed, and that property could only be nuclear charge.

Moseley’s discovery allowed him (in Soddy’s words) to ‘call the roll’ of the elements. No gaps could be allowed in the sequence, only even, regular steps. If there was a gap, it meant that an element was missing.

One now knew for certain the order of the elements, and that there were ninety-two elements and ninety-two only, from hydrogen to uranium. And it was now clear that there were seven missing elements, and seven only, still to be found. The ‘anomalies’ that went with atomic weights were resolved: tellurium might have a slightly higher atomic weight than iodine, but it was element number 52, and iodine was 53. It was atomic number, not atomic weight, that was crucial.

The brilliance and swiftness of Moseley’s work, which was all done in a few months of 1913-14, produced mixed reactions among chemists. Who was this young whippersnapper, some older chemists felt, who presumed to complete the periodic table, to foreclose the possibility of discovering any new elements other than the ones he had designated? What did he know about chemistry – or the long, arduous processes of distillation, filtration, crystallization that might be necessary to concentrate a new element or analyze a new compound? But Urbain, one of the greatest analytic chemists of all – a man who had done fifteen thousand fractional crystallizations to isolate lutecium – at once appreciated the magnitude of the achievement, and saw that far from disturbing the autonomy of chemistry, Moseley had in fact confirmed the periodic table and reestablished its centrality. ‘The law of Moseley…confirmed in a few days the conclusions of my twenty years of patient work.’

Atomic numbers had been used before to denote the ordinal sequence of elements ranked by their atomic weight, but Moseley gave atomic numbers real meaning. The atomic number indicated the nuclear charge, indicated the element’s identity, its chemical identity, in an absolute and certain way. There were, for example, several forms of lead – isotopes – with different atomic weights, but all of these had the same atomic number, 82. Lead was essentially, quintessentially, number 82, and it could not change its atomic number without ceasing to be lead. Tungsten was necessarily, unavoidably, element 74. But how did its 74-ness endow it with its identity?

Though Moseley had shown the true number and order of the elements, other fundamental questions still remained, questions that had vexed Mendeleev and the scientists of his time, questions that vexed Uncle Abe as a young man, and questions that now vexed me as the delights of chemistry and spectroscopy and playing with radioactivity gave way to a raging Why? Why? Why? Why were there elements in the first place, and why did they have the properties they did? What made the alkali metals and the halogens, in their opposite ways, so violently active? What accounted for the similarity of the rare-earth elements and the beautiful colors and magnetic properties of their salts? What generated the unique and complex spectra of the elements, and the numerical regularities which Balmer had discerned in these? What, above all, allowed the elements to be stable, to maintain themselves unchanged for billions of years, not only on the earth, but, seemingly, in the sun and stars too? These were the sorts of questions Uncle Abe had agonized about as a young man, forty years before – but in 1913, he told me, all these questions and dozens of others had, in principle, been answered and a new world of understanding had suddenly opened.

Rutherford and Moseley had chiefly been concerned with the nucleus of the atom, its mass and units of electrical charge. But it was the orbiting electrons, presumably, their organization, their bonding, that determined an element’s chemical properties, and (it seemed likely) many of its physical properties, too. And here, with the electrons, Rutherford’s model of the atom came to grief. According to classical, Maxwellian physics, such a solar-system atom could not work, for the electrons whirling about the nucleus more than a trillion times a second should create radiation in the form of visible light, and such an atom would emit a momentary flash of light, then collapse inward as its electrons, their energy lost, crashed into the nucleus. But the actuality (barring radioactivity) was that elements and their atoms lasted for billions of years, lasted in effect forever. How then could an atom possibly be stable, resisting what would seem to be an almost instantaneous fate?