Question

Why is Hg Mercury a liquid under standard condition?

Answer 1

most of the reading i've seen refers to weak mercury-mercury bonding

Answer 2

There are three factors usually identified that cause Hg atoms to have unusually low tendency to bond to each other:

(1) Hg atoms have completely filled subshells: the 4f, 5d and 6s subshells are all filled. Filled subshells are generally unreactive (cf. the noble gases). Only two other metals share this property, Zn and Cd, and these elements also have unsually low melting points.

(2) The highest energy (valence) electrons in the Hg atom, the 6s electrons, are unusually low in energy (hence less likely to participate in bonding) because, first, the 4f electrons are poor at screening them from the influence of the nucleus, and...

(3) ...second, because the very large kinetic energy these electrons have, in such a large atom, mean relativistic corrections to their mass are significant. Since the electrons act heavier, they spend more time closer to the nucleus (remember the Bohr radius has the electron mass in the denominator). Spending more time closer to the nucleus means the 6s electrons are bound more tightly.

What really needs to be explained is why the metals that are still heavier than Hg, e.g. thallium and lead, are merely low-melting, and not liquid. The answer is that both have unfilled 6p subshells, and these relatively high-energy valence electrons more readily participate in metal-metal bonding, so these metals remain solid at room temperature.