Proton number and nucleon number

All naturally-occurring materials are made up from chemical elements. (hydrogen, helium, etc)

Atoms of different chemical element have a different numbers of protons in their nuclei, and therefore different numbers of electrons in the orbit. Chemical elements are characterized and identified by their atomic numbers and have unique chemical properties.


The proton (or atomic) number is the number of protons present in the nucleus of the atom.

  • The atom is electrically neutral. Hence, the atom has the same number of protons as its number of electrons.


The nucleon (or mass) number is the number of nucleons (sum of protons and neutrons) present in the nucleus of the atom.

  • Since the mass of an atom is concentrated at its nucleus, the nucleon (or mass) number is thus an approximate measure of the mass of the atom.
  • The number of neutrons present in the nucleus of the atom (the neutron number, N) may be deduced by subtracting the proton number (Z) from its nucleon number (A).


Nucleon number (A) = neutron number (N) + proton number (Z)


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