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Atomic number

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The atomic number of an atom is the number of protons in the nucleus of the atom.[1][2] 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.[1] The elements of the periodic table are listed in order of increasing atomic number.[2]

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.


  1. 1.0 1.1 Daintith, John, ed. (2008). A Dictionary of Chemistry (Sixth ed.). Oxford University Press. p. 48. ISBN 978-0-19-920463-2 . 
  2. 2.0 2.1 Moore, John T. (2010). Chemistry Essentials For Dummies. Wiley. p. 36. ISBN 978-0-470-61836-3 .