Electronegativity

Electronegativity

Electronegativity in Group 13 does not fall monotonically down the group. It decreases from boron to aluminium, but then, instead of continuing to fall for gallium, indium and thallium, it rises again slightly for these three heavier members.

This is the same story as atomic radius and ionisation enthalpy: the mismatch in atomic size caused by the poorly-shielding d (and f) electrons in the cores of Ga, In and Tl disrupts what would otherwise be a smooth trend. Because these heavier atoms are smaller/tighter than a simple trend predicts, they hold their valence …