I've always been told in chemistry lessons that lone pairs repel more due to a greater charge density than bonding pairs. And that makes sense when steric effects are involved when bond orbitals overlap.
But this conflicts with what I have learnt in physics i.e. Coulomb's law, which states that the only thing affecting the force between two charged bodies is the magnitude of the charge and the distance between them. Therefore it seems weird to me that a lone pair should "repel" more because of greater charge density, despite having the same charge magnitude as a bonding pair.
What am I neglecting or if I'm right, does that mean for large atoms (such as iodine, where electron pairs are further apart) the lone pairs and bonding pairs repel equally as per Coulomb's law?
Answer
In bonded pairs of electrons, the repulsion of the negative charges is somewhat reduced by the positive charge of the bonded atom's nucleus. Since lone pairs don't have to deal with this positive charge, naturally their repulsion is stronger.
Hope this clarifies your doubts!! :)
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