163. Uncle Tungsten - XVI. Quanta

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Uncle Tungsten

Chapter 24. Brilliant Light

p293 ...Fifty-odd elements were known by 1815; if Dalton was right, this meant fifty different sorts of atoms... 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. 

William Prout (1785-1850) - This was known as Prout's hypothesis. Someone at the turn of the century, learning about radioactive cascades, might imagine the reverse -- that it all started with some super heavy element that broke down into the known elements. 

p294 [Harry Moseley, in 1913, uses X-ray spectroscopy to, as Soddy said,] “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.

p295 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... But Urbain, one of the greatest analytic chemists of all... at once appreciated the magnitude of the achievement... 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.”

...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...
...
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 created radiation in the form of visible 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...
...
p297 It was Niels Bohr, also working in Rutherford’s lab in 1913, who bridged the impossible, by bringing together Rutherford’s atomic model with [Max] 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... Bohr postulated... 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 spectrum 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)... 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, “had been revealed as an atomic music of the spheres.”

It really is impossible to not think of this in musical terms. Especially the jumping from one state to another like jumping from one note to another -- up in pitch and then back down again. This must have been such a wonderful time to be a chemist as the curtain was being drawn back revealing the actual workings of chemistry. Also, I just got why plants only really respond to certain colors (frequencies) of light.

There is a break in the science here as the world is gripped by the Great War. Moseley dies after being shot in the head at Gallipoli. After the war Bohr goes on to make sense of the order of electrons in larger atoms. 


p299 ...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... 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. [H, He] 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. [Li, Be, B, C, N, O, F, Ne] Similarly for the third period or shell...

So to continue the musical analogy, atoms are like musical instruments that can play only a certain range of notes. 


p300 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. [Finally!] The inert gases each had completed outer valency shells with a full complement of eight electrons, and this made them virtually unreactive. [Don’t we need to know the shell order here? (See the video at the end for that info.)] 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. [That seems an odd way of putting it, but I know what he means. I made the mistake of consulting Wiki to see if I could find a way to put it better and discovered (surprise!) it's far more complicated than I imagined. You can click HERE if you want to see what I mean. ] 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. 

This threw me but I finally worked it out: While sodium and chlorine are independently restless because of their relative valency conditions, as soon as they share that electron -- while the salt they form is electrically neutral -- each atom becomes charged since one is now short an electron and becomes positively charged (a cation) as the other gains an electron and becomes negatively charged (an anion). Thus they are electromagnetically bound together by their valency needs. 

Also, when he says above that they become "ionized," in the way I've just described, this is also to say that this is an ionic bond -- as opposed to a covalent bond. I don't think Sacks ever gets around to talking about the covalent bond.  


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. 

Not a clue  

Bohr’s electronic periodic table, based on atomic structure, was essentially the same as Mendeleev’s empirical one based on chemical reactivity... 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. (Footnote: ...Moseley had observed that element 72 was missing, but could not say whether it would be a rare-earth element or not... Bohr, with his clear idea of the number of electrons in each shell, was able to predict that element 72 would not be a rare-earth element, but a heavier analog of zirconium. He suggested that his colleagues in Denmark seek this new element in zirconium ores, and it was swiftly found (and named hafnium, after the old name for Copenhagen). This was the first time the existence and properties of an element were predicted not by chemical analogy, but on the purely theoretical basis of its electronic structure.) Moseley and Bohr had made it absolutely clear that the periodic table was based on a fundamental numeric 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.

Another way of looking at this is to add this series together giving you 2, 10, 18, 36, 54, 86. Which are simply the atomic numbers of Group VII, which Sacks insists on calling "inert" gases though "noble" gases seems to be the preferred usage today. 


...
p302 ...The character and identity of the elements... could now be inferred from their atomic numbers, which no longer just indicated nuclear charge but stood for the very architecture of each atom. It was all divinely beautiful, logical, simple, economical, God’s abacus at work.

What made metals metallic? ... The conductivity of metals was ascribed to a “gas” of free and mobile electrons, easily detached from their parent atoms -- this explained why an electric field could draw a current of mobile electrons through a wire. Such an ocean of free electrons, on the surface of a metal, could also explain its special luster, for oscillating violently with the impact of light, these would scatter or reflect any light back on its own path. 

See also Metallic bonding. 

The electron-gas theory carried the further implication that under extreme conditions of temperature and pressure, all the nonmetallic elements, all matter, could be brought into a metallic state. This had already been achieved with phosphorus in the 1920s, and it was predicted, in the 1930s, that at pressures in excess of a million atmospheres it might be achieved with hydrogen, too -- there might be metallic hydrogen, it was speculated, at the heart of gas giants like Jupiter. The idea that everything could be “metallized” I found deeply satisfying. (Footnote: It was also wondered, early in the twentieth century, what might happen to the “electron gas” in metals if they were cooled to temperatures near absolute zero -- would this “freeze” all the electrons, turning the metal into a complete insulator? What was found, using mercury, was the complete opposite: the mercury became a perfect conductor, a superconductor, suddenly losing all its resistance at 4.2 degrees above absolute zero. Thus one could have a ring of mercury... with an electrical current flowing around it with no diminution, for days, forever.)

p303 ...How could a huge amount of red light be less effective [with fluorescence, phosphorescence, photoelectric cells] than a tiny amount of blue light? It was only after I had learned something of Bohr and Planck that I realized the answer... must lie in the quantal nature of radiation and light, and the quantal states of the atom. Light or radiation came in minimum units or quanta, the energy of which depended on their frequency. A quantum of short-wavelength light -- a blue quantum, so to speak -- had more energy than a red one, and a quantum of X-rays or gamma rays had for more energy still. Each type of atom or molecule -- whether of a silver salt in a photographic emulsion, or of hydrogen or chlorine in the lab, or of cesium or selenium in Uncle Abe’s photocells, or of calcium sufide or tungstate in Uncle Dave’s mineral cabinet -- required a certain specific level of energy to elicit a response; and this might be achieved by even a single high-energy quantum, where it could not be evoked by a thousand low-energy ones.

p304 ...It was only when Uncle Abe showed me his spinthariscope and I saw the individual sparkles in this that I started to realize that light, all light, came from atoms or molecules which had first been excited and then, returning to their ground state, relinquished their excess energy as [a quanta] of visible radiation.  With a heated solid, such as a white-hot filament, energies of many wavelengths were emitted; with an incandescent vapor, such as sodium in a sodium flame, only certain very specific wavelengths were emitted. (The blue light in a candle flame which had so fascinated me as a boy, I later learned, was generated by cooling dicarbon molecules as they emitted the energy they had absorbed when heated.) 

This would appear to be incorrect. Dicarbon only exists at a temperature far hotter than that found in a candle flame. The blue is where the hydrocarbon molecules of the wax are being broken apart into hydrogen and carbon.

Also, what he says above about the spinthariscope is a bit confusing since those "scintillations" are not instances of quantal radiation. Alpha and beta decay are not chemical but radioactive processes. I guess he means that seeing the scintillations made him reflect on the very different process that generated chemical illumination, but this is not particularly clear. 

Sacks can't tell every science story of this amazing period, but there's one that he skips that I need to say something about. Or rather, I'm going to let Wiki tell it: 

At the end of the 1920s Bohr, Heisenberg and Pauli had worked out the Copenhagen interpretation of quantum mechanics, but it was rejected by Planck, and by Schrödinger, Laue, and Einstein as well. Planck expected that wave mechanics would soon render quantum theory—his own child—unnecessary. This was not to be the case, however. Further work only cemented quantum theory, even against his and Einstein's philosophical revulsions. Planck experienced the truth of his own earlier observation from his struggle with the older views in his younger years: "A new scientific truth does not triumph by convincing its opponents and making them see the light, but rather because its opponents eventually die, and a new generation grows up that is familiar with it."^[21] 

What a magnificent scientific debate it was when you consider that Planck, Schrodinger and Einstein were all on the losing side. You can also (almost) see this as a variation of Jonathan Haidt's "elephant." Scientists so buy into their own theories that -- even when these theories break an old paradigm -- the same scientists fight just as doggedly when the next paradigm shift comes along. 


I'm going to take a break here and wrap this, and the book up next time.

HERE'S something I ran into on YouTube (after writing this post) while trying (still without success) to get a better understanding of protonation -- which may yet be the death of me. What I find particularly interesting about this is -- to play my game of transporting people from the past to show them YouTube videos -- if you showed this video to Boyle, Lavoisier, Davy, Faraday, and all the other great figures of chemistry and science we have met here, including especially Mendeleev, they would be astonished and confused and some would probably be hard to convince. None of the "Great Figures" of chemistry (who died much before WW2) had any clue about how chemistry actually worked. Of why the elements behaved as they did. After alchemy was left behind it still took over a century before science could really make sense out of what was happening. And then -- the ultimate joke -- the underlying reality turned out to be as mystical and strange as anything the alchemists could make up. 

We now have "atoms" which can be viewed either as tiny machines or (in the case of hydrogen) a tone or (in the case of the other atoms) a chord that, if left alone, will "work" or "sound" for -- I think the latest estimate is -- quadrillions of years. And as we will see next time, the nucleus of atoms (which Hank Green in that video -- I think mistakenly -- said could be thought of as actual "particles") are, if anything stranger than electrons. The language of QCD would make the craziest alchemist blush.


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