Question

What is order of halogens according to +R effect and why?

Answer 1

Carbon has 2p orbital so has Fluorine. So Overlapping takes place easily and Fluorine donates electrons through back-pi bonding. As the halogens increase in size overlapping gets tougher. Iodine has 5p orbital so it does not overlap with Carbon and fails to donate electron.

so due to the above reason
the order for bezene ring is F>Cl>Br>I