the bond angle changes to 180° the energy of this orbital increases in part because the H-H overlap decreases and in part because the loss of x2p. character decreases the overlap with p+ (Fig. 3.40). The energy of the 1bn orbital is lowered because the His orbitals move into a better position for overlap with the x2p, orbital The weakly antibonding H-H contribution is also decreased. The biggest change occurs for the 2a1 orbital. It has considerable X2s character in the 90o molecule, but correlates with a pure x2p orbital in the 180 molecule. Hence, it shows a steep rise in energy as the bond angle increases. The 1bi orbital is a nonbonding X2p orbital perpendicular to the molecular plane in the 90'molecule and remains nonbonding in the linear molecule. Hence, its energy barely changes with angle.
The principal feature that determines whether or not the molecule is angular is whether 2ai s occupied. This is the orbital that has considerable X2s character in the angular molecule but not in the linear molecule. Hence, a lower energy is achieved if, when it is occupied, the molecule is angular. The shape adopted by an H2x molecule therefore depends on the number of electrons that occupy the orbitals,
The simplest XH, molecule in Period 2 is the transient gas-phase BeH2 molecule," in which there are four valence electrons. These four electrons occupy the lowest two molecular orbitals. If the lowest energy is achieved with the molecule angular, then that will be its shape. We can ecide whether the molecule is likely to be angular by accommodating the electrons hr two orbitalswww