Isotope

An isotope is a variant form of an atom defined by a particular chemical element. Atoms of the same element contain an equal number of protons (positively charged particles in the nucleus), but differ in their number of neutrons (uncharged particles also found in the nucleus). Because the number of protons—called the atomic number—determines the element, all isotopes of an element occupy the same position in the periodic table. The term itself derives from the Greek words isos (same) and topos (place).

Isotopes are conventionally named after the element and the total number of nuclear particles—for example, carbon-12, carbon-13, and carbon-14, or uranium-235 and uranium-238. This total, called the mass number, is the sum of protons and neutrons. The chemical properties of isotopes of the same element tend to be nearly identical, since chemical reactions depend on electrons. Physical properties such as density and nuclear stability, however, differ.

Stable and Radioactive Isotopes

Some isotopes are stable: their nuclei do not decay over a measurable period. Others are radioactive, meaning their nuclei spontaneously transform, emitting radiation and particles. The time required for half of a given quantity of an isotope to decay is called the half-life. Half-lives range from fractions of a second to billions of years. Some elements, such as technetium and uranium, have no stable isotopes.

Atomic Mass and Isotopic Abundance

The atomic mass listed on the periodic table is not the mass of a single atom but a weighted average based on the natural abundances of isotopes. Natural carbon, for instance, consists mostly of carbon-12, with small amounts of carbon-13 and trace amounts of carbon-14. These varying proportions explain why the atomic mass of many elements is not a whole number.

Applications

Isotopes have broad applications. Carbon-14 is used to date organic materials, while uranium and lead isotopes are used to date rocks and the Earth itself. In medicine, radioactive isotopes are employed in medical imaging and cancer treatment. Nuclear power plants depend on separated isotopes such as uranium-235. Atmospheric science and hydrology use stable isotopes of hydrogen and oxygen to track water movement and trace past climate change.

Related Terminology

The term nuclide refers to any nuclear species with a specific number of protons and neutrons. An isobar is a nuclide with the same mass number but a different element; an isotone has the same number of neutrons. A nuclear isomer is a nuclide in an excited energy state at a particular level.