Question

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.

Answer 1

Hydrogen has a higher boiling point than helium due to its greater polarizability, which leads to stronger London dispersion forces. Although helium has a higher molecular weight, its very small and symmetrical electron distribution makes it less polarizable and thus gives it a lower boiling point than hydrogen.

Step-by-step explanation:

The question asks why hydrogen (H2) has a higher boiling point than helium (He) despite helium having a higher molecular weight. The explanation lies in the types of intermolecular forces and their strengths.

While both H2 and He exhibit London dispersion forces, these forces are generally weaker in helium due to its lower polarizability. A helium atom, being very small with its two 1s electrons held close to the nucleus, presents a very symmetrical electron distribution, which is difficult to polarize.

In contrast, hydrogen molecules (H2) have a slightly higher polarizability due to electron-electron repulsions within the molecular orbitals, creating a more significant London dispersion force relative to helium. This results in a higher boiling point for hydrogen compared to helium, despite hydrogen having a lower molecular weight.

Thus, while molecular weight does impact the strength of London dispersion forces, other factors like atomic and molecular size, and polarizability, also play a critical role in determining the boiling points of elements and compounds.