In short
VSEPR theory predicts the shape of a molecule from the idea that electron pairs around the central atom repel one another and so move as far apart as possible.
The name stands for valence shell electron pair repulsion. Count the electron pairs around the central atom, both bonding pairs and lone pairs. The arrangement gives the shape: 2 pairs are linear, 3 are trigonal planar, 4 are tetrahedral, 5 are trigonal bipyramidal and 6 are octahedral. Lone pairs take more room than bonding pairs, so they squeeze the bond angles and change the shape of the molecule.
In the nitrogen has 3 bonding pairs and 1 lone pair, so 4 pairs in all. The pairs point to the corners of a tetrahedron, but the shape of the molecule is trigonal pyramidal, with a bond angle of about .
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