Question

why are hydrogen and helium not liquified at room tempreature.

Answer 1

Bcoz there valency is one so they will not be liquified at room temperature

Answer 2

H and He cant be liquified because their atomic Masses are small . second reason is that intramolecular forces are not present in helium and they cannot form covalent bond London forces are weak than inter molecular forces ,so less energy is required to overcome these forces and gas cannot be cooled.