Sangeetha Pulapaka

 Bond reactivity: C—Cl < C—Br < C—I

Stronger bonds are more difficult to break, making them less reactive. In addition, the reactivity can also be determined by the stability of the corresponding anion formed in solution. One of the many trends on the periodic table states that the largest atoms are located on the bottom right corner, implying that iodine is the largest and chlorine is the smallest. When chlorine leaves as chloride (if it does) in the reaction, it is not so stable compared to iodide. Because there are no resonance forms and inductive stabilizing effects on these individual atoms, the atoms must utilize their own inherent abilities to stabilize themselves. Iodide has the greatest surface area out of these four elements, which gives it the ability to better distribute its negative charge that it has obtained. Chloring, having the least surface area, is much more difficult to stabilize. This is the reason why iodine is the best leaving group out of the three halogens discussed.