Discuss the anomalous behaviour of fluorine.

Explanation

Abnormal behaviour of fluorine : Fluorine being the first element in the group differs considerably form other halogens. The reasons may be traced out to one or more of the following characteristics.
`F_(2)` has
i) a small size
ii) The highest electronegativity amongst the known elements.
iii) No d-orbital are available in its valence shell.
iv) Low enthalpy of F-F bond dissociation and
v) Only two electrons are present in the penultimate shell while other halogens 8 electrons. The abnormal characteristics of `F_(2)` can be summarised as follows.
i) `F_(2)` exhibits - I oxidation state because it is the most electronegative element known. Therefore, no higher oxidation state for it in its compounds.
ii) In it's hydride HF, it has hydrogen bonding, but other halides of hydrogen do not show this property. HF can form `HF_(2)^(-)` ion. No such ions are known with other halogens.
iii) It combines with carbon while others do not combine even under drastic conditions.
iv) `F_(2)` has a lower EA compared to that of `Cl_(2)` even though `F_(2)` is the most electronegative element.
v) Fluorides have the highest ionic character among the halides.
Example : `AlF_(3)` is ionic while `AlCl_(3)` is covalent compound.