The atomic number is the number of protons in the nucleus of an atom, which is also equal to the number of electrons in a neutral atom. It is represented by the symbol Z and is used to uniquely identify ordinary chemical elements. In an ordinary atom that contains protons, neutrons, and electrons, the sum of the atomic number Z and the neutron number N gives the atoms atomic mass number A. Atoms with the same atomic number but different neutron numbers, and hence different mass numbers, are known as isotopes. The atomic number determines how many electrons surround the nucleus, and it is the arrangement of these electrons that determines most of the chemical behavior of an element. In the modern periodic table, the elements are listed in order of increasing atomic number.