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?

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

Found the answer, it is because of size difference that smaller halogens are able to make more effective p orbital overlap with carbon.

  • $\begingroup$ You surely mean that the p-p overlap is more efficient, thus forming the $\pi$ bond? $\endgroup$
    – tschoppi
    Dec 14 '13 at 13:43
  • $\begingroup$ Yeah skipped overlap. Edited. $\endgroup$ Dec 14 '13 at 14:09
  • $\begingroup$ But shouldn't fluorine have a lower urge to use its electron pair for resonance due to smaller radius and higher electronegativity and won't the bond me more polarized in case of fluorine as compared to that of iodine, leading to a weaker covalent bond...? $\endgroup$
    – oshhh
    Feb 19 '17 at 15:20

$\ce{2p-2p}$ 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.


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