Firstly, I'll offer that the article doesn't claim to explain why other elements aren't liquid, and simply observe that it does explain why mercury is: the math works out.
But interestingly there is one other element which is liquid at STP: Bromine. Every element has a melting point, and some element has to have the lowest[1]. If Earth was just a few dozen degrees cooler, we'd have no liquid elements at "STP", a few dozen degrees warmer, and we'd have several liquid elements. So there's nothing particularly special about mercury being the only liquid metal.
Check out this periodic table[2] annotated with melting temperatures. See a pattern? You might argue that the other elements in mercury's column have significantly lower melting points than the metals to the left; indeed Zinc and Cadmium have filled s orbitals as well.
But putting general pattern-ology aside, the macroscopic properties of an atom are essentially completely defined by the orbital structure. Observable properties such as melting point depend on these details in a highly nontrivial way.
[1] Actually many elements are gaseous at STP and have very low "melting" points. Most would actually sublimate at standard pressure.
You're right that the article doesn't mention it explicitly, but it seems to me that the explanation is "large Z and filled 6s orbital yields a liquid at STP if you account for relativity". So I wanted to know why "large Z and filled 6p orbital yields a liquid at STP if you account for relativity" is false.
I think the true explanation is the one you give: STP is totally arbitrary and it's a coincidence that this is the standard we use. That is, my question was a silly one; I didn't notice that this is a coincidence.
However, I think there's a related, more well-formed question:
Br, Hg, and Ga are liquids (or nearly so) at reasonable Earth temperatures (from your [2], Cs is also), but all other elements seem to be solid or gas at reasonable Earth temperatures at 1 atm.
Why is the range where you get a liquid so narrow?
Why do melting temperatures vary so widely?
What sets the scale, and how does it vary over 4 decades (or three decades, excluding H and He---I understand that quantum effects are dominant there at standard pressure)?
EDIT: Would copernicium be a liquid, if its nucleus lived long enough?
Well, my point is that the macroscopic properties of an atom are the result of a quite complicated dependency on the orbital configuration. It's not like the orbitals are legos which you just clunk together to form the atom -- "stick in 4s^2 and you get conductivity, and throw in a 2p^5 for that nice deep red color".
The detailed set of rules that give these results is called quantum mechanics. We have a pretty good idea of the general equation to describe an atomic system (although it's still just an approximation), but we actually don't have closed-form mathematical solutions for many-body systems. The best we can do (except for case of hydrogen, and maybe helium) is to make a numerical simulation, which is what the authors in the article did.
You see, if you throw in even one different electronic orbit, you're liable to radically change the results. If the ingredients are complicated (hundreds of relativistic+quantum-mechanical particles), you're going to get complicated results! By way of analogy, consider how genes encode lifeforms: flip a few CGAT bits and you get cancer; flip some other bits and maybe you'd get some disease resistance.
As to your point about the range of variation -- this is perhaps one of my favorite reasons to study physics! To paraphrase JD Jackson, "Coulomb's law is experimentally known to hold for over 25 orders of magnitude in length scale!" But consider this; the masses involved in the periodic table cover two decades; does this perturb you? The wavelengths of light emitted by a star cover many of orders of magnitude; is that a problem?
Very cool, too bad they listed the melting point temperatures in Celsius instead of Kelvin.
It would be interesting to see it tipped over as a 3-D bar graph (in K). Some of the rows have little dips in them (not just mercury), not just a hump (with carbon at the peak)
It does seem unusual that there is exactly onemetal that is liquid at STP (room temperature? been a long time since chemistry). On the other hand, it's always blown my mind, just a little, that the moon's orbital period and rotational period almost exactly match, so it's not like the universe isn't filled with these weird coincidences, based on relatively simple interactions exploded on macro scales.
Well, the notion that one metal is liquid at our arbitrary reference temperature is a coincidence. But it is no coincidence that the moon's orbital and rotational periods are the same; this is due to a mechanical phenomenon called Tidal Locking[1] and is actually quite common for natural satellites.
An amazing actual coincidence is that the sun and moon appear almost the same size as each other from the surface. The opportunity for such perfect total solar eclipses (with naked-eye visible Baily's Beads) is not common in the universe.
There is at least one short story (the title escapes me) that suggests that our #1 tourist attraction for visiting aliens would be total solar eclipses.
edit: Found it, maybe. This does not sound like the one I read.
> Illegal Alien, by Robert J. Sawyer (1997). An alien visits Earth, supposedly for "research purposes", and observes a total solar eclipse. He then speculates that Earth may be the only planet in the entire Universe whose moon covers its sun perfectly (with only transits or occultations occurring on other planets).
I think my favourite coincidence, are Saturn's moons Epimetheus and Janus.
One of them orbits just below the other and they happen to be in such a position, that as they approach each other, the lower one gains momentum from the mutual gravitational attraction, while the higher one loses momentum. So every four (earth) years, they swap orbits!
When I first heard about that, I just thought it was awesome :-D
Interesting point - but wouldn't Jupiter or Saturn with their many moons also experience solar eclipses in a similar way ? Maybe not as perfect as on Earth, but certainly some of their moons could obstruct the sun?
Those are called transits and occlusions. Very dull and quite frequent.
And gas giants don't have a fixed surface you can stand on. I guess you could find the optimal distance to hover in your space ship to see an eclipse, assuming it is above the cloud layer. But where is the novelty in that?
Plus the sun is much less impressive way out there. Jupiter is 5 times further (Sun appears 0.04 the size). Saturn is 9.5 AU and so the Sun appears 0.011 the size.
Eclipses are still not likely just because you have a bunch of moons. Usually moons will orbit on the planet's plane of orbit. The Earth-Moon system (being more of a binary planet relationship) orbits on the solar plane instead of around the Earth's tilted equator, meaning the Sun/Moon/Earth is vastly more likely to be in alignment for an eclipse.
But interestingly there is one other element which is liquid at STP: Bromine. Every element has a melting point, and some element has to have the lowest[1]. If Earth was just a few dozen degrees cooler, we'd have no liquid elements at "STP", a few dozen degrees warmer, and we'd have several liquid elements. So there's nothing particularly special about mercury being the only liquid metal.
Check out this periodic table[2] annotated with melting temperatures. See a pattern? You might argue that the other elements in mercury's column have significantly lower melting points than the metals to the left; indeed Zinc and Cadmium have filled s orbitals as well.
But putting general pattern-ology aside, the macroscopic properties of an atom are essentially completely defined by the orbital structure. Observable properties such as melting point depend on these details in a highly nontrivial way.
[1] Actually many elements are gaseous at STP and have very low "melting" points. Most would actually sublimate at standard pressure.
[2] http://www.emsb.qc.ca/laurenhill/science/trends2/melting%20p...