Isotopes are forms of an element where the number of neutrons in the nucleus varies, but the number of protons — the atomic number — is the same. This results in different atomic masses but does not affect the atom's charge. All elements have isotopes. For example, elemental carbon has three naturally occurring isotopes. carbon-12, the most common of them, has six neutrons and six protons. The other two, carbon-13 and carbon-14, have seven and eight neutrons, respectively. Carbon-12 and 13 are stable under natural conditions and do not decay into other elements over time. Heavier isotopes, such as carbon-14, have an unstable nucleus and decay into stable products over time by energy emission. These are known as radioactive isotopes. The decay of radioactive isotopes occurs at a constant rate. The decay is the basis for radiometric dating, a technique used to determine geological age.