CBSE Class 12-science Answered

Polarity is distortion of the electron cloud of one atom by another .The tendency of an electron cloud to be distorted from its normal shape is referred to as its polarizability. The polarizability of an ion (or an atom) depends largely on how diffuse or spread out its electron cloud is.As cations become more polarizing (smaller/more
positive) and anions become more polarizable (larger/more negative) melting points and boiling
points decrease. This is because strong lattices have uniform charge distribution. As ions are
polarized, bonding becomes directional. That is, some interactions strengthen while others weaken.
These weakened interactions are easier to disrupt or melt.

Going from He to Xe size of the cation increases and hence polarisibility decreases and hence boiling point increases.