Which of the following AO's can combine to form a MO with a node in the z-direction?
2s
2p
3s
Both a and c
The 2s AO has no nodes, while the 2p AO has a node in the z-direction. Therefore, the 2p AO can combine with another 2p AO to form a MO with a node in the z-direction. The 3s AO has no nodes, so it cannot combine with another 3s AO to form a MO with a node in the z-direction.