The simplest XHZ molecule in Period 2 is the transient gas-phase BeHz molecule,in which there are four valence electrons. These four electrons occupy the lowest two molecular orbitals. lf the lowest energy is achieved with the molecule angular, then that will be its shape. We can decide whether the molecule is likely to be angular by accommodating the electrons in the lowest two orbitals corresponding to an arbitrary bond angle in Fig. 3.38. We then note that the HOMO (the highest occupied molecular orbital) decreases in energy on going to the left of the diagram and that the lowest total energy is obtained when the molecule is linear. Hence, BeHZ is predicted to be linear and to have the configuration 1022. ln CH2, which has two more electrons than BeHZ, three of the molecular orbitals must be occupied. ln this case, the lowest energy is achieved if the molecule is angular and has configuration 123.