 It is multiplied by +1(unchanged) by the identity operation E and by σv(xz) (reflection in the plane bisecting the H-O-H angle).The oxygen atomic orbitals are labeled according to their symmetry as a1 for the 2s orbital and b1 (2px), b2 (2py) and a1 (2pz) for the three 2p orbitals.The two hydrogen 1s orbitals are premixed to form a1 (σ) and b2 (σ*) MO.