Editing Proton (section)

=== Atomic number ===
In [[chemistry]], the number of protons in the [[atomic nucleus|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) [[electron]]s, 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, whereas a Cl<sup>−</sup> [[anion]] has 17 protons and 18 electrons for a total charge of −1.

All atoms of a given element are not necessarily identical, however. The [[number of neutrons]] may vary to form different [[isotope]]s, and energy levels may differ, resulting in different [[nuclear isomer]]s. For example, there are two stable [[isotopes of chlorine]]: {{nuclide|Chlorine|35}} with 35 − 17 = 18 neutrons and {{nuclide|Chlorine|37}} with 37 − 17 = 20 neutrons.