As we learnt in the previous post, simple covalent molecules with higher relative molecular masses (Mr) should have higher m.p./b.p. as compared to those with lower Mr. However, this is not the case for all simple covalent molecules.
When two atoms of different elements are joined together in covalent bonding, the sharing of electrons is not always equal, creating permanent partial charges on each atom. (Fancy A level term = permanent dipole). These types of molecules are known as polar molecules.
Why are the electrons not shared equally?
The atom that exhibits a higher affinity for electrons would “pull” the shared pair of electrons closer to itself, creating a partial negative charge on itself, leaving the other atom with a positive charge.
The relative affinity for electrons in turn depends on the electronegativity of the atom / effective nuclear charge (ENC) of the atom.
Effective Nuclear Charge ≈ No. of protons – No. of inner shell shielding electrons
This is an intuitive formula. The protons in the atom are trying to attract electrons (unlike charges attract) while the inner shell electrons are trying to repel away electrons that are being added to the valence shell (like charges repel). Thus, ENC measures the net attraction that an atom has on the valence electrons.
Using this concept, we calculate the ENC of Hydrogen and Fluorine. As a reminder, Hydrogen has 1 proton (electronic arrangement: 1) while Fluorine has 9 protons (electronic arrangement: 2,7).
ENC of H = 1 – 0 = 1
ENC of F = 9 – 2 = 7
Since Fluorine has higher ENC as compared to Hydrogen, the shared pair of electrons would be pulled closer to the Fluorine atom, leaving the Fluorine atom with a permanent partial negative charge while Hydrogen has a permanent partial positive charge. See diagram below.
It can also be represented like this:
On the other hand, the electrons in a fluorine molecule (F2) are shared equally between the two atoms as the ENC of both atoms are equal. Thus, fluorine molecule will not have any permanent partial positive or partial negative charges. Thus, the intermolecular bonds in Fluorine are the result of the instantaneous-dipole, induced dipole interactions (id-id).
How does all of this explain the difference in b.p./m.p.?
The intermolecular bonds between HF molecules are the result of the electrostatic forces of attraction between the permanent partial positive portion of a molecule with the permanent negative portion of another molecule (fancy A level term permanent dipole-permanent dipole interaction (pd-pd)).
This is stronger than the id-id interactions that exist between the F2 molecules, since in HF, the dipoles are permanent as opposed to fleeting. Since the intermolecular bonds are stronger in HF, they require more energy to break and HF will have a higher m.p./b.p. than F2.