why in group 13 the electronegativity first decreases and then increases?

it is said that electronegativity is greater when the atomic radius is small then why it is so?

1. Electronegativity of an element depends upon:

• The size of the atom:- as the size decreases the electronegativity increases.
• The magnitude of nuclear charge:- as the size decreases the attraction of the nucleus of the atom, towards the shared electron pair incraeses.

Electronegativity decreases from B to Al and then increases, This is due to the discrepancies of atomic sizes of the elements. On going from aluminium to gallium, the electrons start filling into the d-sub shells. Since, these intervening electrons do not screen the nuclear charge; hence the electrons in Ga experience more forces of attraction then in Al causing a decrease in atomic size and increase in electronegativity.

Hope this is clear to you!