It is a theory that describes chemical bonding. VBT states that the overlap of incompletely filled atomic orbitals leads to the formation of a chemical bond between two atoms. The unpaired electrons are shared and a hybrid orbital is formed. The tetrahedral geometry in CH4 is explained by the hybridization of the one 2s orbital and the three 2p orbitals on the carbon atom. Carbon’s four valence electrons occupy the orbitals singly with parallel spins as dictated by Hund’s rule. With this electron configuration, carbon has four half-filled orbitals and can form four bonds with four hydrogen atoms: The geometry of the overlapping orbitals (the hybrids) is tetrahedral, with angles of 109.5° between the orbitals, so the resulting geometry of the molecule is tetrahedral, with 109.5° bond angles. This agrees with the experimentally measured geometry of CH4 and with the predicted VSEPR geometry.
The nitrogen orbitals in ammonia are sp3 hybrids. Three of the hybrids are involved in bonding with three hydrogen atoms, but the fourth hybrid contains a lone pair. The presence of the lone pair lowers the tendency of nitrogen’s orbitals to hybridize. (Remember that the tendency to hybridize increases with the number of bonds formed.) Therefore the bond angle in NH3 is 107°, a bit closer to the unhybridized p orbital bond angle of 90°