Mercury is the only liquid metal at room temperature due to:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Very high ionisation enthalpy and weak metallic bond
The strength of metallic bonding depends on how many electrons each atom gives to the “electron sea”. In the transition metals the (n−1)d electrons also take part, which makes them hard with high melting points. Mercury (5d106s2) has a completely filled d subshell, so no d electrons take part in bonding, and its 6s2 pair is held very firmly (very high ionisation enthalpy). Very few electrons are therefore delocalised, the metallic bonding is weak, and mercury is a liquid at room temperature (m.p. 234 K).
Why the other options are incorrect
Option (2) Low ionisation enthalpy — Incorrect. A low ionisation enthalpy would release electrons easily and give stronger metallic bonding; mercury in fact has a very high first ionisation enthalpy.
Option (3) High atomic mass — Incorrect. Lead and gold are also heavy but are solids; the physical state depends on bonding forces, not on mass.
Option (4) High vapour pressure — Incorrect. This is a result of weak bonding in the liquid, not its cause; in fact the vapour pressure of mercury at room temperature is low.
NCERT (2026-27 edition):
Physical Properties – “… The transition metals (with the exception of Zn, Cd and Hg) are very hard and have low volatility. Their melting and boiling points are high. … The high melting points of these metals are attributed to the involvement of greater number of electrons from (n-1)d in addition to the ns electrons in the interatomic metallic bonding.”