Carbon-12, carbon-13, and carbon-14 are all carbon. Each has six protons. But carbon-12 has six neutrons, carbon-13 has seven, and carbon-14 has eight. They are isotopes, atoms of the same element with different numbers of neutrons. The number of protons defines the element. The number of neutrons defines the isotope. Most elements have multiple isotopes, and some have dozens.

Isotopes behave the same chemically because they have the same number of electrons. But they differ in mass, which affects physical properties slightly and nuclear stability dramatically. Some isotopes are stable. Others are radioactive, decaying over time and emitting radiation. Carbon-14 is radioactive, with a half-life of about 5,730 years. That makes it useful for dating organic remains up to about 50,000 years old.

Isotopes have many uses. Deuterium and tritium are isotopes of hydrogen used in nuclear fusion and as tracers in chemistry. Uranium-235 and uranium-238 are separated for nuclear fuel and weapons. Radioactive isotopes are used in medicine for imaging and therapy. Stable isotopes are used in environmental studies, food authentication, and forensics. The isotope ratio of a sample can reveal its origin, history, and age.