atomic number

An element's atomic number (or proton number, typically symbolized as Z) is the number of protons in a nucleus of that element. (An element is defined to be those atoms with a particular atomic number, e.g., all atoms with atomic number 2 constitute the element helium.) The number of protons determines how many electrons orbit within a neutral atom of the element, a major determinant of the element's chemical properties. Chemistry's periodic table of the elements lays them out in the order their atomic numbers, lining them up in columns such that elements in a column tend to have similar chemical properties.