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.
CHAPTER TWENTY-FOUR
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? Wh
y? 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.
It was Niels Bohr, also working in Rutherford’s lab in 1913, who bridged the impossible, by bringing together Rutherford’s atomic model with Planck’s quantum theory. The notion that energy was absorbed or emitted not continuously but in discrete packets, ‘quanta,’ had lain silently, like a time bomb, since Planck had suggested it in 1900. Einstein had made use of the idea in relation to photoelectric effects, but otherwise quantum theory and its revolutionary potential had been strangely neglected, until Bohr seized on it to bypass the impossibilities of the Rutherford atom. The classical view, the solar-system model, would allow electrons an infinity of orbits, all unstable, all crashing into the nucleus. Bohr postulated, by contrast, an atom that had a limited number of discrete orbits, each with a specific energy level or quantal state. The least energetic of these, the closest to the nucleus, Bohr called the ‘ground state’ – an electron could stay here, orbiting the nucleus, without emitting or losing any energy, forever. This was a postulate of startling, outrageous audacity, implying as it did that the classical theory of electromagnetism might be inapplicable in the minute realm of the atom.
There was, at the time, no evidence for this; it was a pure leap of inspiration, imagination – not unlike the leaps he now posited for the electrons themselves, as they jumped, without warning or intermediates, from one energy level to another. For, in addition to the electron’s ground state, Bohr postulated, there were higher-energy orbits, higher-energy ‘stationary states,’ to which electrons might be briefly translocated. Thus if energy of the right frequency was absorbed by an atom, an electron could move from its ground state into a higher-energy orbit, though sooner or later it would drop back to its original ground state, emitting energy of exactly the same frequency as it had absorbed – this is what happened in fluorescence or phosphorescence, and it explained the identity of spectral emission and absorption lines, which had been a mystery for more than fifty years.
Atoms, in Bohr’s vision, could not absorb or emit energy except by these quantum jumps – and the discrete lines of their spectra were simply the expression of the transitions between their stationary states. The increments between energy levels decreased with distance from the nucleus, and these intervals, Bohr calculated, corresponded exactly to the lines in the spectrum of hydrogen (and to Balmer’s formula for these). This coincidence of theory and reality was Bohr’s first great triumph. Einstein felt that Bohr’s work was ‘an enormous achievement,’ and, looking back thirty-five years later, he wrote, ‘ [it] appears to me as a miracle even today…This is the highest form of musicality in the sphere of thought.’ The spectrum of hydrogen – spectra in general – had been as beautiful and meaningless as the markings on butterflies’ wings, Bohr remarked; but now one could see that they reflected the energy states within the atom, the quantal orbits in which the electrons spun and sang. ‘The language of spectra,’ wrote the great spectroscopist Arnold Sommerfeld, ‘has been revealed as an atomic music of the spheres.’
Could quantum theory be extended to more complex, multi-electron atoms? Could it explain their chemical properties, explain the periodic table? This became Bohr’s focus as scientific life resumed after the First World War.«68»
As one moved up in atomic number, as the nuclear charge or number of protons in the nucleus increased, an equal number of electrons had to be added to preserve the neutrality of the atom. But the addition of these electrons to an atom, Bohr envisaged, was hierarchical and orderly. While he had concerned himself at first with the potential orbits of hydrogen’s lone electron, he now extended his notion to a hierarchy of orbits or shells for all the elements. These shells, he proposed, had definite and discrete energy levels of their own, so that if electrons were added one by one, they would first occupy the lowest-energy orbit available, and when that was full, the next-lowest orbit, then the next, and so on. Bohr’s shells corresponded to Mendeleev’s periods, so that the first, innermost shell, like Mendeleev’s first period, accommodated two elements, and two only. Once this shell was completed, with its two electrons, a second shell began, and this, like Mendeleev’s second period, could accommodate eight electrons and no more. Similarly for the third period or shell. By such a building-up, or aufbau3 Bohr felt, all the elements could be systematically constructed, and would naturally fall into their proper places in the periodic table.
Thus the position of each element in the periodic table represented the number of electrons in its atoms, and each element’s reactivity and bonding could now be seen in electronic terms, in accordance with the filling of the outermost shell of electrons, the so-called valency electrons. The inert gases each had completed outer valency shells with a full complement of eight electrons, and this made them virtually unreactive. The alkali metals, in Group I, had only one electron in their outermost shell, and were intensely avid to get rid of this, to attain the stability of an inert-gas configuration; the halogens in Group VII, conversely, with seven electrons in their valency shell, were avid to acquire an extra electron and also achieve an inert-gas configuration. Thus when sodium came into contact with chlorine, there would be an immediate (indeed explosive) union, each sodium atom donating its extra electron, and each chlorine atom happily receiving it, both becoming ionized in the process.
The placement of the transition elements and the rare-earth elements in the periodic table had always given rise to special problems. Bohr now suggested an elegant and ingenious solution to this: the transition elements, he proposed, contained an additional shell of ten electrons each; the rare-earth elements an additional shell of fourteen. These inner shells, deeply buried in the case of the rare-earth elements, did not affect chemical character in nearly so extreme a way as the outer shells; hence the relative similarity of all the transition elements and the extreme similarity of all the rare-earth elements.
Bohr’s electronic periodic table, based on ato
mic structure, was essentially the same as Mendeleev’s empirical one based on chemical reactivity (and all but identical with the block tables devised in pre-electronic times, such as Thomsen’s pyramidal table and Werner’s ultralong table of 1905). Whether one inferred the periodic table from the chemical properties of the elements or from the electronic shells of their atoms, one arrived at exactly the same point.«69» Moseley and Bohr had made it absolutely clear that the periodic table was based on a fundamental numerical series that determined the number of elements in each period: two in the first period, eight each in the second and third, eighteen each in the fourth and fifth; thirty-two in the sixth and perhaps also the seventh. I repeated this series – 2, 8, 8, 18, 18, 32 – over and over to myself.
At this point I started to revisit the Science Museum and spend hours once again gazing at the giant periodic table there, this time concentrating on the atomic numbers inscribed in each cubicle in red. I would look at vanadium, for example – there was a shining nugget in its pigeonhole – and think of it as element 23, a 23 consisting of 5 + 18: five electrons in an outer shell around an argon ‘core’ of eighteen. Five electrons – hence its maximum valency of 5; but three of these formed an incomplete inner shell, and it was such an incomplete shell, I had now learned, that gave rise to vanadium’s characteristic colors and magnetic susceptibilities. This sense of the quantitative did not replace the concrete, the phenomenal sense of vanadium but heightened it, because I saw it now as a revelation, in atomic terms, of why vanadium had the properties it did. The qualitative and the quantitative had fused in my mind; the sense of ‘vanadiumness’ now could be approached from either end.