111. Solving a chemistry problem

The formation of covalent bonds between two unexcited atoms of chemical elements - electronic analogues: Tl and B is considered.
a) give a graphical explanation of the formation of covalent bonds between these atoms;
b) write down the electronic and valence scheme of the resulting molecule;
c) determine the order (multiplicity) of communication;
d) determine the valence of each element;
e) determine the number of σ - and π -bonds.