The linear combination of atomic orbitals to form molecular orbitals takes place only when the combining atomic orbitals
A. have the same energy
B. have the minimum overlap
C. have same symmetry about the molecular axis
D. have different symmetry about the molecular axis
Choose the most appropriate from the options given below:

Solution

Chemistry ma’am · 4 steps

LCAO needs close energy, same symmetry and maximum overlap: check each statement against these.

1. Same energy. Atomic orbitals combine well only when their energies are the same or very close. A 1s orbital of one atom will not mix with a 2p orbital of the other, because their energies are far apart. So A is correct.
2. Same symmetry. Take the internuclear axis as the z-axis: 2p_z on one atom combines with 2p_z on the other, but 2pₓ cannot combine with 2p_z. With different symmetry about the axis, the positive and negative overlaps cancel to zero. So C is correct and D is wrong.
3. Overlap. The orbitals must overlap as much as possible. The bigger the overlap, the lower the bonding MO sits below the atomic orbitals. Minimum overlap is the opposite of what is needed, so B is wrong.
4. So. Only A and C are correct, which is option (d).

The idea to keep. For LCAO, remember three things: close energy, the same symmetry about the bond axis, and maximum overlap. If a statement breaks any one of these, it is wrong.

Answer from the NTA final answer key. Solution by PrepWiser.