The reasons for the anomalous behaviour of fluorine are as follows:

(i) The smallest size of fluorine

(ii) The highest electronegativity

(iii) Low bond dissociation enthalpy of F-F bond

(iv) Non-availability of d-orbitals in its valence shell

The anomalous properties are as follows;

(i) Fluorine shows only one oxidation state-1 while all other halogens show variable oxidation states like -1, +1, +3, +5, +7(due to small size, high electronegativity)

(ii) Fluorine forms strong hydrogen bonding in its hydrides unlike other halogens. (due to high hydration energy than other elements)

(iii) The compounds of fluorine have higher ionic character than other halogens. (due to high electronegativity forming polar interactions)

(iv) Fluorine has no tendency to form polyhalide ion whereas other halogen form polyhalide ion(due to non-availability of d orbitals in valence shell)