24. Brilliant Light
How 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?
Utterly new principles had to be invoked, or invented, to come to terms with this impossibility. Learning of this was the third ecstasy of my life, at least of my ‘chemical’ life – the first having been learning of Dalton and atomic theory, and the second of Mendeleev and his periodic table. But the third, I think, was in some ways the most stunning of all, because it contravened (or seemed to) all the classical science I knew, and all I knew of rationality and causality.