In chemistry, the term valence measures the number of bonds formed by an atom of a particular element. This depends on the number of valence electrons that are involved in forming valence bonds. For example, an atom which has a valence of one can form one covalent bond. Nitrogen has three valence electrons and can form a maximum of three valence bonds. When observing the periodic table of elements, elements in the main groups 1 and 17 have a valence of 1. Elements in groups 2 and 16, have a valence of 2. Elements in groups 14 and 15 have a valence of 3. And elements in group 14 have a valence of 4. Valence is determined by the number of electrons in the outermost or valence shell.