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When two 1s atomic orbitals (one from each atom) combine to form molecular orbitals, which of the following statements is correct?

1. The bonding molecular orbital (BMO) is formed by constructive overlap and has lower energy than the atomic orbitals
2. The antibonding molecular orbital (ABMO) is formed by constructive overlap and has higher energy
3. The gerade (symmetric) combination is given by: psi_g = psi_A + psi_B
4. The ungerade (antisymmetric) combination is given by: psi_u = psi_A + psi_B

Correct answer: The gerade (symmetric) combination is given by: psi_g = psi_A + psi_B

Solution

The in-phase combination psi_A + psi_B produces the bonding sigma MO, which has gerade symmetry (symmetric under inversion through the bond centre), so psi_g = psi_A + psi_B. The out-of-phase combination psi_A - psi_B is ungerade and antibonding.

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