Compound

Summary: A substance made of two or more different elements chemically combined in fixed proportions. Tags: igcse chemistry definition atomic-structure Created: 2026-07-14 Last Updated: 2026-07-14

A compound is a pure substance consisting of two or more different elements chemically combined by bonds (ionic or covalent) in a fixed, definite proportion by mass that cannot be altered by physical means. The atoms within a compound are held together by strong chemical bonds: ionic compounds involve the electrostatic attraction between oppositely charged ions formed by electron transfer between metal and non-metal atoms, while covalent compounds involve the sharing of electron pairs between non-metal atoms to achieve stable noble gas electronic configurations. A fundamental principle is that compounds exhibit physical and chemical properties entirely different from those of their constituent elements — for example, sodium chloride (NaCl) is an edible white crystalline solid essential for life, yet sodium is a soft, violently reactive metal and chlorine is a toxic yellow-green gas. Compounds must be clearly distinguished from mixtures, where the components are not chemically bonded, are not present in fixed proportions, and can typically be separated by physical techniques such as filtration, distillation, or chromatography. On the IGCSE exam, students should know that compounds can only be broken down into simpler substances or into their constituent elements by chemical reactions (such as thermal decomposition or electrolysis), not by physical separation methods.

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