Identify the central atom in NH\_3, which is nitrogen (N).
Determine the number of valence electrons around the nitrogen atom. Nitrogen has 5 valence electrons, and each hydrogen contributes 1 electron, for a total of 8 electrons (or 4 electron pairs) around nitrogen.
Use the VSEPR (Valence Shell Electron Pair Repulsion) theory to predict the electron-pair geometry. According to VSEPR, electron pairs (bonding and lone pairs) arrange themselves to minimize repulsion.
Count the bonding pairs and lone pairs on nitrogen. NH\_3 has 3 bonding pairs (with hydrogen atoms) and 1 lone pair on nitrogen, making a total of 4 electron pairs.
Since there are 4 electron pairs around nitrogen, the electron-pair geometry is tetrahedral, even though the molecular shape (based on atoms only) is trigonal pyramidal.