Question

Describe anomalous behaviour of fluorine with the other elements of group $17$ with reference to:
Hydrogen bonding.

Answer 1

Anomalous behaviour of fluorine with the other elements of group $17$: Fluorine shows anomalous behaviour with the other elements of group $17$ due to : $\left(1\right)$ small size, $\left(2\right)$ high electronegativity and $\left(3\right)$ non-availability of d-orbitals in its valence shell.
(a) Hydrogen bonding: On account of high electronegativity of fluorine atoms, extensive hydrogen bonding occurs in $HF$ molecule while it is absent in $HCl,HBr$ and $HI$. The presence of hydrogen bonding in $HF$ is explained as under:
$\left(1\right)HCl,HBr$ and $HI$ are gaseous while $HF$ is a liquid with an abnormally high boiling point.
$\left(2\right)$ $HF$ forms a number of compounds containing $H{F}_2^{-}$ ion (e.g. $KH{F}_2$) while such compounds are not given by other $HX$ molecules.
$H-FH-F\underset{\underset{H-bond}{\uparrow }}{H-F}(HF{)}_n$