CBSE Class 10 Answered

Mercury is a liquid at room temperature because the "relativistic contraction" of its atomic orbitals makes it behave chemically almost like a noble gas, not wanting to share electrons with other atoms, even other mercury atoms! Note that mercury is monatomic in the gas phase, just like a noble gas. Of course this should not be pushed too far; mercury is not a noble gas. There are enough outer-electron interactions for mercury to remain a liquid (radon, 22 amu heavier, is a gas), to conduct electricity, and to participate in ordinary chemical reactions.