Explain the nature of the $C-X$ bond in alkyl halides.

Halogen atoms are more electronegative than carbon; therefore,the carbon-halogen bond of an alkyl halide is polarized. The carbon atom bears a partial positive charge $(\delta+)$ whereas the halogen atom bears a partial negative charge $(\delta-)$. The bond is represented as $C^{\delta+}-X^{\delta-}$.
As we go down the group in the periodic table,the size of the halogen atom increases. The fluorine atom is the smallest and the iodine atom is the largest. Consequently,the carbon-halogen bond length increases from $C-F$ to $C-I$.
Carbon-Halogen $(C-X)$ Bond Lengths,Bond Enthalpies,and Dipole Moments:

Bond	Bond length $/ pm$	$C-X$ Bond enthalpies $/ (kJ \ mol^{-1})$	Dipole moment $/$ Debye
$CH_3F$	$139$	$452$	$1.847$
$CH_3Cl$	$178$	$351$	$1.860$
$CH_3Br$	$193$	$293$	$1.830$
$CH_3I$	$214$	$234$	$1.636$

Order of dipole moment: $CH_3Cl$ > $CH_3F$ > $CH_3Br$ > $CH_3I$
Order of bond enthalpies: $CH_3F$ > $CH_3Cl$ > $CH_3Br$ > $CH_3I$