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NH3 Molecular Orbitals
Ammonia has the following symmetry elements: E, 2C3, 3σv. These symmetry elements classified Ammonia as a C3v point group.

The character table of C3v point group are shown below:

Reducible Representation
The 2s and 2Pz orbital individually transforms as A1 irreducible representation, while 2Px and 2Py both transforms as E irreducible representation.

Symmetry-Adapted Linear Combinations (SALCs)
SALC A1 = $${1 \over \sqrt{3}} (\Phi_1 + \Phi_2 + \Phi_3) $$

SALC E(1) = $${1 \over \sqrt{3}} (2\Phi_1 - \Phi_2 - \Phi_3) $$

Degeneracy
Ammonia has a doubly degenerate (E) orbital. Pauli exclusion principle dictates that any two electron can not have the same quantum state and any exchange must be antisymmetric, therefore the second SALC must be orthogonal to the first SALC (E).

SALC E(23) = $${1 \over \sqrt{2}} (\Phi_2 - \Phi_3) $$

Energy Levels
Each SALCs has different energy level. The node of each SALC roughly indicate how much energy is in the bond. The Nitrogen atom in ammonia can only participate in σ and π (Px and Py) bonding, whereas the Hydrogen atoms can only participate in σ bonding. The non-bonding, 2Pz (a1), orbital give raise to the lone pair on Nitrogen atom in Ammonia.