Why are most metals solids? The ionization energy of metallic elements is very low. They easily lose their valence electrons, and share them with surrounding metal atoms. This delocalized 'soup' of valence electrons binds together the metal cations and gives metals their characteristic properties. The mobility of electrons in the soup explains metal's ability to conduct electricity and heat. Metals are workable because cations can slide past each other but still be bound by the electron soup. And the more valence electrons added to the soup, the stronger and more solid the metal will be. Surrendering more valence electrons produces a higher positive charge on cations, and higher negative charge on the soup- and very strong bonding of the metal atoms.
Mercury hangs on to its valence 6s electrons very tightly. Mercury doesn't contribute its valence electrons readily to the soup. The thinner soup can't bind the mercury atoms together very strongly. Mercury atoms easily slip past and away from each other. Heat easily overcomes the weak binding between mercury atoms, and mercury boils and melts at lower temperatures than any other metal. Because the valence electron soup is thinner for mercury, its electrical and thermal conductivity are poor.
Why is the pair of 6s electrons so inert? The s electrons are able to come very close to the nucleus. They sling 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 much 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 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|
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.
Author: Fred Senese firstname.lastname@example.org
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Last Revised 02/23/18.URL: http://antoine.frostburg.edu/chem/senese/101/inorganic/faq/print-why-is-mercury-liquid.shtml