# Order of positive mesomeric effect of halides

I learned that the order of $+M$ effect (mesomeric effect) for halogens is as follows: $\ce{F} > \ce{Cl} > \ce{Br} > \ce I$.

I wonder why this is so?

• Can you briefly describe what should than be observable result of such effect? – ssavec Dec 13 '13 at 20:44
• I am confused why would Florine being more EN and smallest will have the highest tendency to donate lone pair than Iodine. – Ayush Pateria Dec 14 '13 at 8:06

• You surely mean that the p-p overlap is more efficient, thus forming the $\pi$ bond? – tschoppi Dec 14 '13 at 13:43
$\ce{2p-2p}$ orbitals overlaps better than $\ce{2p-3p}$, $\ce{2p-4p}$, $\ce{2p-5p}$. Although $\ce{F}$ is more electronegative, but it is also smaller in size. Both $\ce{F}$ and $\ce{C}$ (next to it) have $\ce{2p}$ orbitals to overlap.