The atomic number or proton number (symbol 'Z') of a chemical element is the number of protons found in the nucleus of an atom. It is identical to the charge number of the nucleus. The atomic number uniquely identifies a chemical element. In an uncharged atom, the atomic number is also equal to the number of electrons.
The atomic number ''Z'', should not be confused with the mass number ''A'', which is the total number of protons and neutrons in an atom. This number of neutrons, ''N'', completes the weight: ''A'' = ''Z'' + ''N'' (all are whole numbers). Since protons and neutrons have approximately the same mass (and the mass of the electrons is negligible for many purposes) and the mass defect of nucleon binding is always small compared to the nucleon mass, the atomic mass of any atom is roughly equal to the whole number ''A''.
Atoms with the same atomic number ''Z'' but different neutron numbers ''N'', and hence different atomic masses, are known as isotopes. A little more than three-quarters of naturally occurring elements exist as a mixture of isotopes, and the average isotopic mass of an isotopic mixture for an element (called the relative atomic mass) in a defined environment on Earth, determines the element's standard atomic weight.
Adopted from http://wikipedia.org/atomic_number