Nature of the C--X Bond in Haloalkanes

Fluorine, chlorine, bromine and iodine are all more electronegative than carbon, so in every haloalkane the $\text{C--X}$ bond is a polar covalent single bond with the carbon carrying a partial positive charge and the halogen a partial negative charge: $\text{C}^{\delta+}{-}\text{X}^{\delta-}$. This permanent bond dipole is the feature that makes the halogen-bearing carbon electrophilic and hence susceptible to attack by nucleophiles -- it is the reason haloalkanes undergo nucleophilic substitution and elimination reactions at all.

Two related but distinct trends govern how this bond changes as the halogen changes down the group $\text{F} \to \text{Cl} \to \text{Br} \to \text{I}$:

Bond length increases. Fluorine is the smallest halogen and iodine the largest, so the $\text{C--X}$ internuclear distance increases steadily: $\text{C--F} \approx 1.39\ \text{\AA}$, $\text{C--Cl} \approx 1.78\ \text{\AA}$, $\text{C--Br} \approx 1.93\ \text{\AA}$, $\text{C--I} \approx 2.14\ \text{\AA}$ (typical values for a saturated haloalkane).

Bond enthalpy (strength) decreases. Even though fluorine forms the shortest bond, it is also the strongest: $\text{C--F}$ is the strongest carbon-halogen single bond known, and bond strength falls steadily to $\text{C--I}$, the weakest. This is because bond strength here is controlled mainly by the effectiveness of orbital overlap between carbon's relatively small $2p$ orbital and the halogen's valence orbital -- the size mismatch grows as the halogen gets larger (down to iodine's much larger $5p$ orbital), so overlap, and hence bond strength, falls even as the halogen's own size makes the bond geometrically longer. This declining bond strength is exactly why alkyl iodides are the most reactive of the haloalkanes towards substitution (the $\text{C--I}$ bond is easiest to break) even though the $\text{C--F}$ bond is by far the most polar. …