In chemistry the number of protons in the nucleus of an atom is known as the atomic number, which determines the chemical element to which the atom belongs. For example, the atomic number of chlorine is 17; this means that each chlorine atom has 17 protons and that all atoms with 17 protons are chlorine atoms. The chemical properties of each atom are determined by the number of (negatively charged) electrons, which for neutral atoms is equal to the number of (positive) protons so that the total charge is zero. For example, a neutral chlorine atom has 17 protons and 17 electrons, while a negative Cl− ion has 17 protons and 18 electrons for a total charge of −1.
All atoms of a given element are not necessarily identical, however, as the number of neutrons may vary to form different isotopes, and energy level may differ forming different isomers. For example, there are two stable isotopes of chlorine: 3517Cl and 3717Cl.
