← Bonding, structure, properties and energy changes

The four solid types, and conductivity

Every paper asks you to classify a solid and explain its conductivity — the rule is always the same, only the substance changes.

Solid type	Particles	Attractive forces	Conducts as a solid?
Ionic	Ions	Ionic bonds (electrostatic)	No (ions fixed in the lattice) — YES molten or dissolved
Metallic	Metal atoms/cations	Metallic bonding (delocalised electrons)	Yes — delocalised electrons are free to move
Covalent network	Atoms	Covalent bonds throughout	Usually no (diamond) — EXCEPT graphite (see below)
Molecular	Molecules	Intermolecular forces (London, dipole-dipole, H-bonding)	No — no charged particles are free to move at all

The single rule underneath every conductivity question: a substance must contain CHARGED PARTICLES that are FREE TO MOVE. State this general rule first, then apply it to the specific substance.

Worked example Diamond versus graphite conductivity Diamond is an electrical insulator; graphite conducts electricity. Both are covalent networks of pure carbon. Explain the difference. DIAMOND: each carbon atom is covalently bonded to 4 others in a 3-D tetrahedral network. Every valence electron is locked into a covalent bond, so there are no free-moving charged particles — diamond does not conduct. GRAPHITE: each carbon atom is covalently bonded to only 3 others, arranged in 2-D hexagonal layers. This leaves ONE delocalised electron per carbon, free to move throughout each layer. Those delocalised electrons ARE the free-moving charged particles the general rule requires, so graphite conducts electricity even though it is a covalent network, like diamond. ✓ The difference is the number of bonds each carbon forms (4 vs 3) — that difference determines whether every electron is locked in place (diamond) or one is left delocalised and mobile (graphite). ✕

"Covalent network" does not mean "does not conduct" — check for delocalised electrons specifically

Graphite is the standard exception that catches students who memorise "covalent networks do not conduct" as a blanket rule. The actual test is always the SAME general rule (free-moving charged particles), applied carefully to the specific bonding — diamond fails that test because every electron is locked into a bond, while graphite passes it because one electron per carbon is left delocalised. State the general rule, then show your working for the specific substance, rather than quoting a memorised exception list. ℹ

Solid or molten changes the answer for ionic and metallic solids

An ionic solid does NOT conduct as a solid (ions are fixed in the lattice), but DOES conduct once molten or dissolved, because the ions then become free to move. A metallic solid conducts in BOTH states, because its delocalised electrons are already free to move regardless of whether the metal is solid or liquid. A question specifying "as a solid" versus "molten" is testing exactly this distinction.

Practise just this