Two nuclides are isotones if they have the same number N of neutrons. This is to be contrasted with:

• Isotopes are nuclides having the same number of protons
• Isobars are nuclides having the same mass number, i.e. sum of protons plus neutrons.
• Nuclear isomers are different excited states of the same type of nucleus. A transition from one isomer to another is accompanied by emission or absorption of a gamma ray, or the process of internal conversion. (Not to be confused with chemical isomers.)