= Solution
The hydrogen $A_1$ <symmetry orbital> mixes with both <nitrogen> $2s$ and $2p_z$, giving three $a_1$ <molecular orbitals>. The two hydrogen $E$ functions mix with $(2p_x,2p_y)$, giving a bonding $e$ pair and an antibonding $e$ pair. Eight valence <electrons> fill the lower levels as
$$
\boxed{(2a_1)^2(1e)^4(3a_1)^2.}
$$
This uses conventional numbering that reserves $1a_1$ for the omitted <nitrogen> core $1s$ orbital. The low $2a_1$ and $1e$ levels provide the principal N–H <bonding molecular orbitals>; the highest occupied $3a_1$ level is the mainly nitrogen <lone pair>, with some bonding admixture permitted by the pyramidal geometry. Empty $4a_1$ and $2e$ levels are antibonding. Symmetry fixes which orbitals can mix, rather than their quantitative energy separations.
\Image[/past-exam-of-the-chemistry-a-course-of-the-university-of-cambridge/2021/paper-1-ammonia-orbital-correlation.png]
{title=Original qualitative <ammonia molecular orbital diagram> comparing pyramidal and planar geometries; the core orbital is omitted}
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