Isobar (nuclear)

A nuclear isobar are when two nuclear species exhibit the same total number of nucleons for each nucleus, but the number of protons and neutrons making up that total are different. In contrast, a nuclear isomer would be two nuclear species that have the same number of protons and neutrons, but the elements vary in energy state. An isobar is similar to an isotope or isotone in that it describes a different atomic nucleus with similarities.[1] Isobars always have the same mass number (which is the number written to the upper left of the chemical symbol).

Some examples of nuclear species that are isobars include:

  • [math]\ce{ ^{40}_{20}Ca}[/math] and [math]\ce{ ^{40}_{19}K}[/math]
  • [math]\ce{ ^{14}_{6}C}[/math] and [math]\ce{ ^{14}_{7}N}[/math]

In the above examples, [math]\ce{ ^{40}_{20}Ca}[/math] and [math]\ce{ ^{40}_{19}K}[/math] are isobars since they both have 40 nucleons in their nuclei. The same is true for [math]\ce{ ^{14}_{6}C}[/math] and [math]\ce{ ^{14}_{7}N}[/math] , which each have 14 nucleons. A daughter nucleus after beta decay will always be an isobar of the parent nucleus, since it doesn't change the total number of nucleons in the nucleus the way alpha decay does. Note that different isobars must always have a different number of protons (different atomic number), and must therefore be different chemical elements.

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. Changing the number of neutrons in the nucleus changes the isotope.

For Further Reading


  1. A.D. McNaught and A. Wilkinson (Ed.). (2014, Dec. 12). IUPAC Compendium of Chemical Terminology (2nd ed., the "Gold Book") [Online]. Available: