The atomic number of an atom is the number of protons in the nucleus of the atom. The atomic number of an atom defines which element it is. In a neutral atom, the atomic number is equal to the number of electrons orbiting the nucleus. The elements of the periodic table are listed in order of increasing atomic number.
Atomic number is not the same as:
- atomic mass (symbol: ma), which is the mass of a single atom, commonly expressed in unified atomic mass units
- mass number (symbol: A), which is the sum of the number of protons and number of neutrons in the nucleus of an atom
- relative atomic mass (also called atomic weight; symbol: Ar, ), which is the ratio of the average mass per atom of an element from a given sample to 1/12 the mass of a carbon-12 atom.
Atomic number is not changed by the number of electrons or neutrons; it is simply the proton number.