Isotope

Summary: Atoms of the same element with the same number of protons but different numbers of neutrons. Tags: igcse chemistry definition atomic-structure Created: 2026-07-14 Last Updated: 2026-07-14

Isotopes are atoms of the same element that contain the same number of protons (and therefore the same atomic number) but differ in their number of neutrons, giving them different mass numbers. Because isotopes of an element have identical electron configurations, their chemical properties are virtually the same — carbon-12, carbon-13, and carbon-14 all form CO₂ upon combustion and participate in the same reactions. Physical properties that depend on mass, such as density, rate of diffusion, and melting and boiling points, vary slightly between isotopes. The relative atomic mass (Aᵣ) listed on the periodic table is a weighted average of the masses of all naturally occurring isotopes, taking their abundances into account, and IGCSE candidates must be able to calculate Aᵣ from isotopic abundance data. Carbon-14 is radioactive and decays with a half-life of 5730 years, forming the basis of radiocarbon dating — an application often cited in exam questions linking chemistry to real-world science.

Key term — IGCSE Chemistry (0620/0971) glossary.