The atomic number of an element is the number of protons in the nucleus of an atom of that element. It is represented by the symbol Z. The atomic number can be used to uniquely identify ordinary chemical elements. In an ordinary uncharged atom, the atomic number is also equal to the number of electrons. For 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 mass of an element is the total mass of an atom and is typically expressed in atomic mass units (amu). An atoms atomic mass will be very close to its mass number, but will have some deviation in the decimal places.