Delocalised Electrons – IGCSE Chemistry Definition

Delocalised electrons are electrons that are not fixed to or associated with a single atom or bond. Instead, they are free to move throughout the entire structure. In IGCSE Chemistry, delocalised electrons are most important in two contexts: metallic bonding and the structure of graphite.

In metals

When metal atoms pack together, each atom releases its outer-shell electrons. These electrons are no longer attached to any one atom. They form a “sea” of delocalised electrons that flows around and between the positive metal ions. The electrostatic attraction between the positive ions and this electron sea is metallic bonding.

The delocalised electrons explain three key properties of metals. First, electrical conductivity: when a potential difference is applied, the delocalised electrons drift through the structure, carrying current. Second, thermal conductivity: the mobile electrons transfer kinetic energy rapidly from hot regions to cooler regions. Third, malleability: when layers of ions slide past each other, the delocalised electrons redistribute to maintain the bonding, so the metal bends rather than shatters.

In graphite

In graphite, each carbon atom forms three covalent bonds with neighbouring carbon atoms, creating flat hexagonal layers. The fourth outer electron from each carbon atom is delocalised and free to move along the layers. This is why graphite conducts electricity, unlike diamond, where all four outer electrons of each carbon are fixed in covalent bonds.

Exam context

A frequent Paper 4 question asks candidates to explain why metals conduct electricity or why graphite conducts but diamond does not. The answer must include the phrase “delocalised electrons” or “free electrons” and state that they are free to move and carry the charge.

Worked exam question

Explain why copper is a good electrical conductor. (2 marks)

Copper has delocalised electrons / a sea of free electrons [1]. These electrons are free to move through the metal structure and carry electrical charge / current [1].

Common mistakes

Candidates sometimes write “free ions” instead of “free electrons.” Ions in a solid metal are not free to move --- they vibrate in fixed positions. Only the electrons are delocalised. Another error is saying graphite has “metallic bonding.” Graphite has covalent bonding within layers and weak forces between layers, plus delocalised electrons --- but the bonding is not metallic.