If we compare the boiling point of hydrogen and helium using molecular weight criteria (both have London dispersion forces as intermolecular forces of attraction because both are non polar then the one which have which have higher molecular weight will have higher intermolecular attraction forces) then helium should have greater boiling point, but if we see the boiling point data for H2 and helium then we found that H2 have its boiling point as approximately 20 Kelvin while He will have approx 4.3 Kelvin.


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Higher molecular weight is not the determining factor. Rather the number of electrons that could be polarized and the volume of space over which they may be polarized are the key factors in dispersion forces.

For species with similar structures higher molecular weight goes along with more or larger atoms, thus more electrons and greater polarizability; but "monatomic" and "diatomic" are not really similar structures. Compared with helium, hydrogen has as many electrons (two), and the presence of two atoms instead of one allows an opportunity for polarization over more volume. So hydrogen will have more dispersion forces.


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