Arrange H2, D2 and T2 in the decreasing order of their boiling points and heat of fusion. (NO LINKS)

Dear student,

(a) Boiling point:
As the molecular weight increases the boiling point increases because smaller molecules have greater thermal motion, on average than big molecules because kinetic energy is proportional to mass and the square of velocity.
The order: 
$H_2 < D_2 < T_2$
Hydrogen has the lowest weight than the other two.

(b) The heat of fusion: Bigger the molecule larger would be the energy required to break the molecule.
Order: $H_2 < D_2 < T_2$

Regards