In simple terms

Isotopes are versions of the same element. They carry the same number of protons, which is what makes them that element, but different numbers of neutrons.

The International Atomic Energy Agency puts it simply: isotopes share almost the same chemical properties but differ in mass.

How it works

An isotope is named by its total count of protons plus neutrons. Carbon occurs naturally as carbon 12 with six neutrons, carbon 13 with seven, and carbon 14 with eight.

Some isotopes are stable and emit nothing. Others are unstable, known as radioisotopes, and give off radiation as they decay. The US Department of Energy counts 254 stable isotopes, and every artificially produced isotope is unstable.

Unstable isotopes decay at a fixed statistical pace, described by their half-life. Carbon 14 has a half-life of about 5,730 years, which is what makes radiocarbon dating possible.

Hydrogen is the only element whose isotopes carry their own names. Deuterium has one neutron and tritium has two.

Why it matters

Isotopes of an element behave almost identically in chemistry but differ in mass, so they can be told apart and followed. That turns them into a measuring tool rather than a curiosity.

The Department of Energy lists nuclear medicine, oil and gas exploration, basic research and national security among their uses.

Where you’ll see it

• Radiocarbon dating of archaeological finds
• Medical imaging and cancer treatment
• Oil and gas exploration
• Nuclear fuel and safeguards inspections

Example

An archaeologist dates a wooden post by measuring how much carbon 14 remains in it, then using the 5,730 year half-life to work back to roughly when the tree died.

Often confused with

Isotopes are not ions. Isotopes differ in neutron count, so they differ in mass. Ions differ in electron count, so they differ in charge.

Editorially reviewed by Specialty Digest Editorial TeamLast reviewed September 16, 2026Researched and drafted with AI assistanceReport an issue