\(\mathop {\underline {R - I\, > \,R\, - \,Br\, > \,R\, - \,Cl\, > \,R\, - \,F} }\limits_{\;{\text{ }}increasing{\text{ }}bond{\text{ }}energy} \)

   decreasing halogen reactivity.

This order depends on the carbon-halogen bond energy; the carbon-fluorine bond energy is maximum and thus fluorides are least reactive while carbon - iodide bond energy is maximum hence iodides are most reactive.