Tl belongs to group 13, elements of this group have the outer electronic configuration of s2p1. Monovalency is explained by the 's' electrons in the outer shell remaining paired, and not participating in bonding. this called 'inert pair effect'.If the energy required to unpaired them exceeds the energy evolved when they form bonds, then the 's' electrons will remain paired. the strength of MX3 compounds decreases down the group.The mean bond energy for chlorides is TlCl3 = 153KJ/mol. Thus the 's' electrons are most likely to be inert in thallium.