Question

Why the melting and boiling point of helium is lower than krypton? (4marks)

Answer 1

Because helium's intermolecular forces of attraction are weaker than krypton's IFA.

Explanation:

Force of attraction increases with the size of the atom because the polarizability of an atom increases as the atomic number increases.

This means that because a helium atom is smaller than a krypton atom, the IFA of helium must be weaker than the IFA of krypton.

Melting/boiling points is the energy required to break the bonds to change the state of a given molecule/atom. If helium's bonds are weaker, less heat/energy is required to break the bonds thus melting/boiling points are lower!

Answer 2

It is; well perhaps you should quote some data..

Explanation:

Helium has a normal boiling point of `4.2*K`, and a lower melting point. Krypton has a melting point of `116*K`, and a normal boiling point of `121*K`.

So why? The Noble gases are not molecular they are ATOMIC....and the only intermolecular force that can operate are dispersion forces which are based on the polarization of the electron cloud. And since krypton has more electrons than helium...many more in fact...(`He`, `Z=2`; `Kr`, `Z=36`), krypton should be more involatile given the greater degree of dispersion forces that operate between the molecules....