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Refer to the following webpage :
http://antoine.frostburg.edu/chem/senese/101/periodic/faq/why-is-mercury-liquid.shtml
《Why is mercury a liquid at STP?》

"Mercury is puzzling in several ways. It's a liquid at room temperature and pressure, but all of its neighbors on the periodic table are solids. Mercury is much less reactive than cadmium or zinc. It's difficult to oxidize, and it doesn't conduct heat or electricity as well as other members of its group.

Why are most metals solids?
Most metals share their valence electrons with surrounding metal atoms. Picture the metal as a lattice of positive ions glued together by "sea" of shared valence electrons. This electron sea model explains many properties of metals. For example, metals conduct electricity because the shared electrons are free to move about; taking an electron from one part of the metal will cause electrons from surrounding areas to rush in and fill the hole. Metals can be drawn into wire or pounded into sheets because the metal ions can slide past each other but still be bound together by the shared valence electrons.
The electron sea model explains some trends in metal hardness and melting point. Harder, high-melting-point metals tend to share more valence electrons than softer, more easily melted metals. For example, magnesium has a higher melting point than sodium because Mg2+ centers are glued together by an electron sea with 2 electrons for every atom, while each atom in sodium metal contributes only one electron.

Mercury hangs on to its valence 6s electrons very tightly.
Mercury-mercury bonding is very weak because its valence electrons are not shared readily. (In fact mercury is the only metal that doesn't form diatomic molecules in the gas phase).
Heat easily overcomes the weak binding between mercury atoms, and mercury boils and melts at lower temperatures than any other metal. The thin valence electron sea makes mercury's ability to conduct electricity and heat much poorer than expected for a metal at that position in the periodic table.

Why is the pair of 6s electrons so inert?
The s electrons are able to come very close to the nucleus. They swing around very massive nuclei at speeds comparable to that of light. When objects move at such high speeds, relativistic effects occur. The s electrons behave as though they were more massive than electrons moving at slower speeds. The increased mass causes them to spend more time close to the nucleus. This relativistic contraction of the 6s orbital lowers its energy and makes its electrons much less likely to participate in chemistry- they're buried deep in the atomic core.

Why doesn't this effect make gold and thallium liquid too?
Let's compare the electronic configurations for gold, mercury, and thallium:
Atom.....Average atomic mass ...... Ground state configuration
Au ............196.9665 ................ [Kr] 4d10 4f14 5s2 5p6 5d10 6s1
Hg ........... 200.59 ....................[Kr] 4d10 4f14 5s2 5p6 5d10 6s2
Tl ........... 204.383 ................... [Kr] 4d10 4f14 5s2 5p6 5d10 6s2 6p1
All three atoms have very low energy 6s orbitals. But the gold 6s orbital is only half filled. Accepting an electron into that low energy orbital will lower energy overall, and metal-metal bonding is expected to be strong as a result. Still, the 6s electron is held tightly and gold's reputation as a 'noble metal' comes from its inertness.
Thallium is more massive, so the 6s pair is even more inert than in Hg. But thallium has a 6p electron. Remember that p electrons can't approach the nucleus as closely as s electrons; the p orbital has a nodal plane that passes through the nucleus. So that 6p electron is fairly reactive compared to the 6s electrons. That explains why the most common ion of thallium is Tl+, and not the +3 ion like B and Al and other members of its family.