Generaly, elements in Group 12 all have unusually low boiling points in comparsion to their neighbours (ofcourse with indium exception). This is connected with their electron configuration. Their density is significantly lower too, especially for mercury. This in combination with arrangement in solid structure (hexagonal Zn, Cd and rhombohedral Hg) leads to low melting point of mercury (you can compare values of heat of fusion and heat of vaporization - both are very low). The unusuality of mercury can be observed in its behavior in gaseous phase where resembles noble gases - its mainly monoatomic. This again shown low tendency of mercury atoms to aggregate - this can be explained more better by quantum physics (sorry not my business).
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)
Ref.
D. S. Rustad, "How soft is mercury? (Letter to the Editor)", Journal of Chemical Education, 64, 470 (1987)
The electrons in a mercury atom are bound more tightly than usual to the nucleus. In fact, the s electrons are moving so fast and close to the nucleus they exhibit relativistic effects, behaving as if they were more massive than slower-moving electrons. It takes very little heat to overcome the weak binding between mercury atoms. Because of the behavior of the valence electrons, mercury has a low melting point, is a poor electrical and thermal conductor and doesn't form diatomic mercury molecules in the gas phase.
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