Atoms of the same element that have different atomic masses due to having different numbers of neutrons in the nuclei, but which still have similar chemical properties. Many of these forms of elements with a specific number of electrons (such as carbon 14 or potassium 40) are unstable and decay into different elements, releasing their surplus electrons. Radiocarbon, potassium-argon, fission track, and thermoluminescence dating all rely on this phenomenon in different ways.