Question

why mercury is not evaporating as it is a liquid

Answer 1

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 diatomicmercury molecules in the gas phase. 

Answer 2

It can happen when liquids are cold or when they are warm.... It turns out that all liquids can evaporate at room temperature and normal air pressure. Evaporation happens when atoms or molecules escape from the liquid and turn into a vapor. Not all of the molecules in a liquid have the same energy.
♪ ╭∩╮（︶︿︶）╭∩╮

hope it will helps mate