What is an isotope?

Each element has a unique number of protons. For example, every carbon atom contains 12 protons. The number of neutrons can differ for each "form" of the same element - this is known as an isotope. For example, carbon-12 contains 12 protons and 12 neutrons in the nucleus, carbon-13 contains 12 protons and 13 neutrons, and carbon-14 contains 12 protons and 14 neutrons. Isotopes therefore have different relative atomic masses, and may differ in radioactivity. Carbon-14 is radioactive, but carbon-12 and carbon-13 are not.