Octet Rule

Summary: Atoms tend to gain, lose, or share electrons to achieve 8 electrons in their outer shell (noble gas configuration). Tags: igcse chemistry definition atomic-structure bonding Created: 2026-07-14 Last Updated: 2026-07-14

The octet rule states that atoms tend to gain, lose, or share electrons in order to attain a full outer shell of eight electrons, achieving the stable electronic configuration of the nearest noble gas. This drive toward a full octet is the fundamental reason atoms form chemical bonds: metals in Groups 1 and 2 lose electrons to reach the configuration of the preceding noble gas, while non-metals in Groups 16 and 17 gain electrons to reach the configuration of the following noble gas. The rule explains the stoichiometry of many compounds — sodium (2.8.1) loses one electron and chlorine (2.8.7) gains one electron, so they combine in a 1:1 ratio to form NaCl, whereas magnesium (2.8.2) loses two electrons and combines with two chlorine atoms to give MgCl₂. Important exceptions exist, however: hydrogen and helium are stable with only two outer electrons (the “duet” rule), and elements such as boron and aluminium can be stable with six electrons in their outer shell. IGCSE students should apply the octet rule when drawing dot-and-cross diagrams for ionic and covalent bonding, always checking that each atom (except hydrogen) is surrounded by eight electrons in the final structure.

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