The oxidation number for NH3, also known as ammonia, is 0. Although the oxidation number of the nitrogen atom in the molecule is -3, the oxidation numbers for the three hydrogen atoms are equal to +3, so the two elements cancel each other out.
Oxidation numbers are assigned to atoms that form polar covalent bonds. Covalent bonds are formed when an atom shares the electrons in its outermost orbit with an adjacent atom. The sharing of electrons results in a more stable state for the atoms, so they are less reactive and remain bonded together in a molecule.
Oxidation numbers indicate the number of electrons and spaces available for sharing in the atoms' respective orbits. A positive oxidation number indicates that the atom has extra spaces to take in electrons, while a negative oxidation number indicates that the atom has extra electrons to share. These create negative and positive charges. In the case of ammonia, the electrons are distributed evenly such that the charges on the atoms cancel each other out, resulting in an overall neutral molecule.