Glossary
isotope
Isotopes are atoms of the same element with different numbers of neutrons, so they share chemistry but differ in mass.
Isotopes are atoms of one element that differ only in their number of neutrons. They have the same atomic number, so they are the same element and behave the same way chemically; what changes is the mass number, and sometimes the stability of the nucleus. Carbon exists as carbon-12 with six neutrons and carbon-14 with eight, and it is the second that decays and is used for dating. The convention is to write the mass number after the name or as a superscript before the symbol. Because the chemistry is identical, a question cannot ask which isotope reacts faster — the difference that matters is weight or radioactivity. The atomic mass printed on the periodic table is a weighted average across the isotopes that occur naturally, which is why it is rarely a whole number: chlorine's 35.45 is the blend of chlorine-35 and chlorine-37.
Taught in
Related terms
- atomic number — An element's atomic number is the number of protons in its nucleus, and it is what decides which element the atom is.
- ion — An ion is an atom or group of atoms with a charge, made by losing electrons (positive cation) or gaining them (negative anion).
Source: NIST periodic table and reference data for the atomic masses and half-life figures, NIH PubChem for the substance data, and NIST Special Publication 811 for the unit conventions — the same sources the chemistry guide cites. All three are US federal government material, not subject to copyright in the United States. Every definition, example and calculation above was written for this course; no dictionary or textbook text is reproduced. (NIST and PubChem accessed 2026-10-26; definitions written 2026-10-27)