The anomalous behaviour of fluorine is due to its:
1. Small size
2. highest electronegativity
3. low F-F bond dissociation enethalpy
4. non-availability of d-orbitals in its valence shell.
The two examples are:
1. Due to non-availability of d-orbitals in its valence shell, fluorine cannot expand its octet, therefore, shows only -1 oxidation state while all other halogens due to the presence of d-orbitals shows positive
2. Due to its small size, the three lone pair of electrons on each F atom in F – F molecule, repel the bond pair. As a result, F – F bond dissociation energy is lower than that of Cl – Cl bond.