Nuclear species with the same number of neutrons in the nucleus but a different number of protons (which makes it a different element) are called isotones of that element.[1]

All matter is made up of atoms in various arrangements. Atoms are composed of protons, neutrons, and electrons. Each proton has a charge of [math]+1[/math], each electron a charge of [math]-1[/math], and each neutron has no charge (neutral charge, giving it its name 'neutron'). When nuclear scientists study different nuclear species, it's useful to look at nuclei with the same number of neutrons, but changing the number of protons changes the chemical nature of the nucleus. This concept of isotones isn't used nearly as often as the related concept of isotope, which have the same number of protons but a different number of neutrons. The mnemonic for remembering the two concepts is that isotone has an n, for neutron, while isotope has a p for proton.

An example to illustrate the point would be different isotones with 20 neutrons, for example: [math]\ce{^{36}_{16}S}[/math], [math]\ce{ ^{37}_{17}Cl}[/math], [math]\ce{^{38}_{18}Ar}[/math], [math]\ce{^{39}_{19}K}[/math], and [math]\ce{ ^{40}_{20}Ca}[/math]. There are a lot of stable nuclear species with 20 neutrons because 20 is a magic number of neutrons.[2]

Phet: Build a Nucleus

The Energy education team has adapted the following simulation from the University of Colorado. This simulation shows how neutrons and protons sit in energy levels and make up the nucleus. The number of neutrons and protons maintain particular ratios for the nucleus to be stable.


  1. McNaught, A.D. and A. Wilkinson (Eds.). (2014, Dec. 10). IUPAC. Compendium of Chemical Terminology (2nd ed., the "Gold Book"). Available:
  2. (2014, Dec. 10). Table of Nuclides [Online]. Available: