OK, let’s talk about something with pretty much no practical relevance whatsoever: the element copernicium. That’s #112, just below mercury in the periodic table, and its longest-lived isotope has a half-life of 29 seconds. Which is actually pretty impressive – that’s one of the longest-lived elements up there at those atomic weights, and it’s long enough that if you look smart you can actually study its properties and its chemistry a bit. Why would anyone want to do that (“Because I have a grant” is not the answer we’re looking for here)?
Well, it’s because of how strange mercury is. Like the properties of water, the properties of mercury actually have come to seem weirder and weirder to me as I learn more chemistry and physics. (This feeling isn’t limited to chemical properties, by the way – for example, when I was a kid, I took things like elephants and giraffes for granted. Sure, there they were, that’s a giraffe all right. Now they seem pretty close to extraterrestrial; I’m amazed to see them walking around). But water is just a bizarre substance in every way, a freakish sticky mass of hydrogen bonds, and mercury’s not far behind it. Think about it: what is a metallic element down there in that row of the periodic table doing as a liquid at room temperature? Not even solidifying until cold-front-in-Alberta temperatures, at that? It’s freakish, and the explanation for it is rather unnerving, too.
For something to be a gas at a given temperature, the forces that would otherwise condense it to a liquid state have to be weak enough to allow it, and for something to be a liquid, the forces that would otherwise make it a solid have to be weak enough for that not to happen, either. So pure mercury being a liquid at room temperature means that the interatomic attractions between mercury atoms must be abnormally small.
Some of the properties of mercury (and of copernicium) are due to “lanthanide shielding“, and that is at least understandable in a classical mental-picture way. The lanthanides (and higher elements beyond them) have atoms with smaller radii than you’d predict from just following the trends earlier in the periodic table. But that’s because those atom sizes have to do with the attraction of the outermost electrons to the nucleus (negative and positive charges), and that attraction is partly “shielded” by the inner electrons in the way. That effect diminishes as you lay on more electrons, though: the d and f electron orbitals are progressively less effective at shielding, and the lanthanide elements generally get smaller as you go up. This same effect is responsible, among other things, for making hafnium a lot more like zirconium, one row up, than you’d figure it would be, and separating out really pure hafnium took quite a bit of work.
But the bigger effect is relativistic. That’s actually a notable example of Paul Dirac being completely wrong about something in physics – he had stated back in 1929 (PDF here if you’re up for it!) that relativistic corrections to quantum mechanics were of “no importance” because they would apply only to very high-speed particles (that is, those moving at an appreciable fraction of the speed of light). But as it turns out, the inner electrons of the heavier elements are moving at such speeds (they get faster as the positively charged nucleus gets bigger and more charged), and this has effects out to the chemically important outer electrons as well. For one thing, relativistic particles are heavier, and this actually shrinks the atomic radius of the heavier elements still more and has complex effects on the various orbitals.
In the 1960s and 1970s these effects began to be more appreciated. Mercury’s outermost electrons were believed to be much more involved than they would normally be in interactions with the nucleus, and thus much less involved in attraction with other mercury atoms. But it wasn’t until 2013 that Peter Schwerdtfeger (Massy Univ., New Zealand) and colleagues at other centers were able to nail down that the exact contribution to mercury’s melting point. (I may have mentioned this before, but I’ve long thought that a book titled “Quantum Mechanics: A Hand-Waving Approach” would sell quite well in the textbook market). Without relativistic corrections, mercury’s melting point is predicted to be 82C, rather than -39. (These calculations, direct quantum-mechanical influence on bulk melting point, are extremely painful, which is why it took until the 21st century for hardware and algorithms to be up to the task).
And now Schwerdtfeger and colleagues have turned to copernicium. Those effects that make mercury’s outer electrons less attentive to the outside world would be predicted to be even stronger in copernicium, leading to predictions in the 1970s that it would be practically inert. But in 2008, experimental evidence came in that the element (which at the time was unnamed!) had more metallic character than expected, via interaction with a gold surface. The new calculations, though, which were presumably even more computationally intensive than those needed for mercury, strongly suggest that this result was due to dispersion forces. Copernicium, the authors believe, almost certainly has noble-gas characteristics and indeed may only barely be a liquid at room temperature (!) The new paper refers to it as a “relativistic noble liquid”, which is quite a weird category – it’s even more like mercury than mercury is.
Relativistic quantum effects are also the reason for gold being yellow and for lead-acid batteries being able to work at all (since we were discussing batteries around here the other day!) No one makes batteries out of tin, and that’s what lead would be like, electrochemically, if it weren’t for the relativistic changes. I find the whole intersection of the two fields very interesting – not least because this is special relativity crossing with quantum mechanics as opposed to general relativity (both of which get referred to commonly as just “relativity”) And there’s an interesting point: quantum mechanics predicts extremely counterintuitive and unusual phenomena, which have been observed exactly as predicted and to extraordinary levels of accuracy. Special relativity, likewise: it predicts wild things which have been experimentally verified from a great many different angles and in extreme detail. So it’s perhaps no surprise that the two get along fine. General relativity as well makes crazy-sounding predictions which also have stood up perfectly to every single experimental test, starting with the bending of light in the 1919 eclipse and going on to this day with the observation of gravitational waves. But general relativity and quantum mechanics. . .oh, boy.
When it comes to gravity, the two theories are completely incompatible, and there is no way to escape the conclusion that one or both of them must be seriously incomplete or even flat-out wrong about something important. There is no quantum theory of gravity as yet, despite huge amounts of brainpower being expended on the problem. A new understanding is out there somewhere, one that encompasses both of these hugely successful and powerful current theories and then shows something even larger and more powerful behind them both. And we don’t know what it is. The disputes about what the next physics will look like have attained near-religious intensity (and money has changed hands more than once), and who knows when it will ever be resolved?