CH3-Cl has a shorter bond length compared to CH3-I. The bond length is influenced by the size of the atoms or groups bonded together. In this case, iodine (I) is larger than chlorine (Cl) in terms of atomic size.
As you move down a group in the periodic table, the atomic size generally increases due to the addition of new electron shells. Therefore, iodine (I) is larger than chlorine (Cl).
The larger size of iodine in CH3-I leads to increased electron-electron repulsion within the molecule. To minimize this repulsion, the bond length between the carbon (C) and iodine (I) atoms increases, causing the CH3-I bond to be longer compared to CH3-Cl.
On the other hand, chlorine (Cl) is smaller in size, resulting in less electron-electron repulsion and a shorter bond length in CH3-Cl.