Chemical Bonding

Why do atoms bond?

Atoms bond to achieve a full outer shell of electrons (usually 8 — the octet rule; 2 for hydrogen and helium). This gives a more stable, lower-energy arrangement like that of a noble gas.

Ionic bonding

Ionic bonds form between a metal and a non-metal. The metal atom transfers one or more electrons to the non-metal atom, forming oppositely charged ions. These ions are held together by strong electrostatic attraction between opposite charges.

Example — sodium chloride (NaCl):

• Na (2,8,1) loses 1 electron → Na⁺ (2,8) — noble-gas configuration
• Cl (2,8,7) gains 1 electron → Cl⁻ (2,8,8) — noble-gas configuration

Ionic compounds form a giant ionic lattice: a regular 3-D arrangement of alternating positive and negative ions. This structure gives high melting points and the ability to conduct electricity when molten or dissolved (ions are free to move).

Dot-and-cross diagrams for ionic compounds show electron transfer with only the outer shells drawn; cross electrons go to the anion.

Covalent bonding

Covalent bonds form between two non-metal atoms. Each atom contributes one electron to a shared pair that counts towards the full outer shell of both atoms.

• Single bond: one shared pair (e.g. H–H in H₂, H–Cl in HCl)
• Double bond: two shared pairs (e.g. O=O in O₂, C=O in CO₂)
• Triple bond: three shared pairs (e.g. N≡N in N₂)

Dot-and-cross diagrams for covalent molecules show shared pairs in the overlap region; show all outer electrons.

Examples of CCEA required molecules: H₂O (2 bonding pairs + 2 lone pairs on O), NH₃ (3 bonding pairs + 1 lone pair on N), CH₄ (4 bonding pairs, no lone pairs on C), HCl, Cl₂, CO₂, O₂, N₂.

Metallic bonding

In metals, atoms lose their outer electrons to form a lattice of positive ions surrounded by a sea of delocalised electrons (free electrons). The strong electrostatic attraction between the positive ions and the electron sea holds the metal together.

This explains:

• High melting points: strong electrostatic forces require a lot of energy to break.
• Electrical conductivity: delocalised electrons carry charge through the lattice.
• Thermal conductivity: delocalised electrons transfer kinetic energy rapidly.
• Malleability/ductility: layers of ions can slide over each other without breaking bonds (electrons re-bond immediately).

Summary of bond types

CCEA exam tips

• Always label donor and acceptor in ionic dot-and-cross diagrams.
• In metallic bonding questions, use the phrase "delocalised electrons" — "free electrons" alone scores no marks at Higher.
• Draw lone pairs explicitly on covalent diagrams — NH₃ and H₂O without lone pairs lose marks.