Three types of strong bond

There are three types of strong chemical bond. Which one forms depends on the kinds of element involved:

• Ionic — between a metal and a non-metal. Electrons are transferred.
• Covalent — between non-metals only. Electron pairs are shared.
• Metallic — between metal atoms. A sea of delocalised electrons holds positive ions together.

Watch out: the difference between ionic and covalent is transfer vs sharing. Ionic = electrons given away/taken to form ions; covalent = electrons shared between atoms. Never say covalent bonds "transfer" electrons.

Predict the bond

Ionic bonding: electron transfer

When a metal reacts with a non-metal, the metal atoms lose their outer electrons and the non-metal atoms gain them. Both end up with a full outer shell (a noble-gas arrangement):

• Metals (Groups 1, 2, 3) lose electrons → positive ions (+1, +2, +3).
• Non-metals (Groups 6, 7) gain electrons → negative ions (−2, −1).

Dot-and-cross tip: use dots for one atom's electrons and crosses for the other's, so the examiner can see where each electron came from. The ion is drawn in square brackets with its charge outside.

Which ion forms?

Tap the ion each atom forms when it bonds ionically. Think about the nearest full shell.

Giant ionic lattices

Ionic compounds are not made of molecules. The ions pack into a regular, repeating 3-D pattern — a giant ionic lattice — held by strong electrostatic forces of attraction acting in all directions between oppositely charged ions.

Why high melting points? There are many strong electrostatic bonds, so a large amount of energy is needed to break them — giant ionic lattices have high melting and boiling points.

Conducting electricity

Covalent bonding: shared pairs

When non-metal atoms bond, neither atom can give electrons away — so they share. Each covalent bond is a shared pair of electrons, giving both atoms a full outer shell.

Covalent gives three kinds of structure: tiny small molecules (H₂O, CH₄, CO₂); long-chain polymers; and giant covalent structures (diamond, graphite, silicon dioxide) where billions of atoms are joined.

Small molecules & polymers

Small molecules (like CH₄) have strong covalent bonds inside the molecule, but only weak intermolecular forces between molecules. It is those weak forces that are overcome when they melt — so small molecules have low melting and boiling points and are often gases or liquids.

Polymers are very large molecules: many small units joined by covalent bonds into long chains. The intermolecular forces between these long chains are relatively strong, so polymers are solids at room temperature.

Name that structure

Diamond vs graphite

Both are made of only carbon, joined by strong covalent bonds into a giant covalent structure — yet their properties are completely different because the carbon atoms are arranged differently.

Why does graphite conduct? Each carbon forms only 3 bonds, so one electron per carbon atom is delocalised (free to move). These free electrons carry charge — that's why graphite conducts electricity and heat, just like a metal. Diamond has no free electrons, so it cannot conduct.

Graphene, fullerenes & C₆₀

• Graphene — a single layer of graphite, just one atom thick. It is strong, conducts electricity (delocalised electrons) and is used in electronics and composites.
• Fullerenes — molecules of carbon shaped as hollow tubes or cages, mostly hexagonal rings.
• Buckminsterfullerene (C₆₀) — the first fullerene discovered; a hollow ball of 60 carbon atoms.
• Carbon nanotubes — cylindrical fullerenes with a very high length-to-diameter ratio; used for their strength and conductivity (e.g. in nanotechnology and electronics).

Diamond or graphite?

Metallic bonding

Metals are a giant structure of positive metal ions arranged in regular layers, surrounded by a 'sea' of delocalised electrons (the outer-shell electrons, shared across the whole structure). The strong attraction between the positive ions and this electron sea is the metallic bond.

Why do metals conduct & bend? The delocalised electrons are free to move, so they carry electric charge and thermal energy — metals are good conductors. The layers of ions can slide over each other, so metals are malleable (can be bent and shaped). Strong metallic bonds mean high melting points too.

Why alloys are harder

In a pure metal the atoms are the same size and sit in neat layers that can slide easily — which makes pure metals soft. An alloy is a mixture of a metal with other elements. The different-sized atoms distort the layers, so they can no longer slide over each other easily — making alloys harder.

Why bother with alloys?

Three states & the particle model

The particle model represents particles as small solid spheres. It explains the three states and the changes between them:

The state symbols in equations are (s) solid, (l) liquid, (g) gas and (aq) aqueous (dissolved in water). The stronger the forces between particles, the more energy needed to melt or boil — so the higher the melting and boiling points.

Limitations of the model: it assumes particles are solid, inelastic spheres with no forces between them — but real particles aren't all spheres, and there are forces between them (which is exactly why melting and boiling happen).

Reading the model

Bonding-type sort

Tap a substance, then tap the bonding type that holds it together.

The sizes of particles

Particles are classified by size:

• Coarse particles (dust) — 1×10⁻⁵ m to 2.5×10⁻⁶ m (2 500–10 000 nm).
• Fine particles — 100 nm to 2 500 nm (1×10⁻⁷ to 2.5×10⁻⁶ m).
• Nanoparticles — 1 nm to 100 nm across, only a few hundred atoms. (1 nm = 1×10⁻⁹ m.)

Key idea: nanoparticles have a huge surface-area-to-volume ratio. This means a tiny amount can have a large active surface, so they often have different properties from the same material in bulk — and you may need far less of them.

Uses and risks

Because of their huge surface area and unusual properties, nanoparticles have many uses:

• Catalysts — large surface area means more reactions per gram.
• Sun creams — give better UV protection and don't leave a white film.
• Medicine — can deliver drugs deep into the body.
• Electronics, cosmetics & deodorants (e.g. silver nanoparticles kill bacteria).

Risks & ethics: because nanoparticles are so new, their long-term effects on health and the environment are not fully known. They may be able to enter cells or be breathed in, so their use needs careful testing — a genuine risk-vs-benefit judgement.

Your turn — surface area : volume

Structure ↔ property

Tap a structure on the left, then the property it explains on the right.

Explain the property

The whole of 4.2 in one place

You've covered the whole of AQA 4.2 — Bonding, structure and the properties of matter. Press Finish to see your score.