Question

why do the covalent radii not increase regularly from b to tl?​

Answer 1

The covalent radius, rcov, is a measure of the size of an atom that forms part of one covalent bond. It is usually measured either in picometres (pm) or angstroms (Å), with 1 Å = 100 pm. In principle, the sum of the two covalent radii should equal the covalent bond length between two atoms, R(AB) = r(A) + r(B).

Answer 2

Melting and boiling point trend of boron family - definition.........

The M.P. and B.P. of boron family do not show regular trend, this is probably due to unusual crystal structures of B to Ga. Actually melting point decreases sharply on moving down the group from B to Ga then increases Ga to Tl.