Question

Why there is unusual change in the order of electronegativity of boron family(13 group)?

Answer 1

The electronegativity first decreases from B to Al and then increases down the group.

Reason: As we move from Boron to aluminium ,the atomic size increases ,considerably, therefore attraction of nucleus for the electron decreases and hence electronegativity decreases. As the size increases but the effective nuclear charge increases due to poor shielding of the inner of the inner d and f electrons. As a result, force of attraction of the nucleus for the electrons increases and hence the electronegativity increases from Al to Tl.