Bonding, structure and the properties of matter — section overview

Section C2 explains why substances have their properties by examining how their atoms are bonded together and how they are arranged structurally.

The three types of bonding

Bond type	Between	Electron behaviour	Example
Ionic	Metal + non-metal	Electron transferred	NaCl, MgO
Covalent	Non-metal + non-metal	Electrons shared	H₂O, CH₄
Metallic	Metal atoms	Delocalised sea of electrons	Fe, Cu, Al

Ionic bonding

Atoms lose/gain electrons to achieve full outer shells, forming ions.

Metal atoms → lose electrons → positive ions (cations) Non-metal atoms → gain electrons → negative ions (anions)

Ionic compounds form giant ionic lattices — strong electrostatic forces between oppositely charged ions in all directions.

Properties of ionic compounds:

• High melting and boiling points (strong electrostatic forces)
• Conduct electricity when molten or dissolved (ions free to move), not when solid (ions fixed)
• Soluble in water (usually)

Covalent bonding

Atoms share electrons to achieve full outer shells.

Simple molecular compounds (e.g. H₂O, CO₂, CH₄):

• Low melting and boiling points (weak intermolecular forces)
• Do not conduct electricity (no ions or delocalised electrons)

Giant covalent structures (e.g. diamond, graphite, silicon dioxide):

• Very high melting points (strong covalent bonds throughout)
• Diamond: each C bonded to 4 others; very hard; does not conduct
• Graphite: layers of hexagons; delocalised electrons between layers → conducts electricity; layers slide → lubricant

Metallic bonding

Metal cations in a lattice surrounded by a sea of delocalised electrons.

Properties of metals:

• Good conductors of electricity and heat (delocalised electrons)
• Malleable and ductile (layers of ions can slide over each other)
• High melting points (strong metallic bonding)

Alloys

Alloys (e.g. steel, bronze) contain atoms of different sizes, disrupting regular lattice → harder than pure metals.

Polymers

Long-chain molecules with repeating units. Thermosoftening: weak forces between chains → soften on heating. Thermosetting: cross-links → rigid even when heated.

Common exam mistakes in C2

1. Covalent bonds strong; intermolecular forces weak — simple molecules have LOW boiling points because intermolecular forces (not bonds) break; bonds remain intact
2. Graphite conducts; diamond does not — graphite has one delocalised electron per carbon; diamond has none
3. Ionic compounds — conduct only when molten or dissolved — solid ions are in fixed positions