The electron pair geometry of methane and ammonia is tetrahedral. Both have four pairs of electrons around the central atom (C and N). In methane, C has four bonds (4 BP). In ammonia, N has 3 bonds (3 BP) and one lone pair of electrons (1 LP).Electron pair geometry is determined by the number of electrons around the central atom, both bond pairs (BP) and lone pairs (LP). These geometry is a domain electrons postulated by VSEPR.
This domain is as a result of minimizing the repulsions between electron pairs. Remember, in the VSEPR theory, the repulsion of BP - BP < BP - LP < LP - LP. In practise, the balloon for LP is bigger than BP, because the repulsion of LP is stronger than BP.
Because methane has 4 BP, the strength of repulsion is the same, its molecular geometry is also tetrahedral, as same as its electron pair geometry. Ammonia has 3BP and 1 LP, the repulsion of LP - BP > BP - BP. That is why, its molecular geometry is trigonal pyramidal. So the molecular geometry differs with its electron pair geometry. Please think about water molecule and compare its geometry with methane and ammonia.
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