Kr has higher boiling point because it has a higher molecular weight (84 amu vs 40 amu). Consider the size pf the atoms and the interatomic forces (since the noble gases exist as single atoms) involved. The only forces are van der Waal’s (or London Dispersion) forces. These are a function of the polarizability of the atom, which is determined by diffusion of the electron cloud. The size of the atoms increases as we go down the group in Periodic table. It also increases as number of electrons increases. Hence, the electron cloud gets bigger, and the polarizability also increases. The electrons can move around easily so there will be instantaneous dipole moments (one side will be + and the other side will be -) which will also result in greater van der Waal’s/London Dispersion forces. Therefore, Kr has higher boiling point than Argon.
