As molecular mass increases, van der Waals interactions between molecules also increase, because numbers of electrons in the molecules.
Iodine (I) has greatest atomic number, so the number of electrons.
Hydrogen fluoride is exception, because hydrogen bonding between molecules.
Hydrogen bond is an electrostatic attraction between two polar groups that occurs when a hydrogen atom (H), covalently bound to a highly electronegative atom such as flourine (F), oxygen (O) and nitrogen (N) atoms.
