Question
Considering x–axis as the internuclear axis which out of the following will not form a sigma bond and why? (a) 1 s and 1 s (b) 1 s and 2px : (c) 2py and 2py (d) 1s and 2s.
Solution
Sigma bond is always formed between two half-filled atomic orbitals along their internuclear axis.i.e the line joining the centres of the nuclei of two atoms(axial overlapping). 2py and 2py will not form a sigma bond because taking x-axis as the internuclear axis, there will be lateral (sideway) overlap between the two 2pv orbitals forming a 
bond.
bond.
