Bonding, Structure and the Properties of Matter — AQA Combined Science: Trilogy

This unit explains how atoms join together by chemical bonding, the structures that form, and how those structures explain the properties of materials.

The three states of matter

• Solid — particles in a regular fixed arrangement, vibrating in place; fixed shape and volume.
• Liquid — particles close together but able to move; fixed volume, takes the shape of its container.
• Gas — particles far apart, moving quickly in all directions; fills its container.

The particle model explains changes of state. Melting, boiling, freezing and condensing are physical changes (no new substance). The stronger the forces between particles, the higher the melting and boiling points. A limitation of the simple model is that it treats particles as solid spheres with no forces between them.

Three types of bonding

Chemical bonds form so atoms achieve a full outer shell of electrons. The type of bonding depends on the elements involved:

• Ionic — between a metal and a non-metal.
• Covalent — between non-metals.
• Metallic — between metal atoms.

Ionic bonding

Atoms transfer electrons: the metal loses electrons to form a positive ion (cation); the non-metal gains electrons to form a negative ion (anion). The oppositely charged ions are held together by strong electrostatic forces of attraction in all directions — this forms a giant ionic lattice.

Properties of ionic compounds:

• High melting and boiling points — strong forces need lots of energy to break.
• Conduct electricity when molten or dissolved (ions are free to move), but not when solid (ions are fixed).

Dot-and-cross diagrams show which atom each electron came from.

Covalent bonding

Atoms share pairs of electrons. Each shared pair is a covalent bond. Covalent substances form two main structures:

Small molecules (simple molecular)

e.g. H₂, Cl₂, H₂O, CO₂, CH₄.

• Low melting and boiling points — the intermolecular forces (between molecules) are weak and easily broken, even though the covalent bonds inside molecules are strong.
• Do not conduct electricity — no free electrons or ions.

Polymers

Very large molecules made of many repeating units joined by covalent bonds. They are usually solid at room temperature because of the larger intermolecular forces.

Giant covalent structures

Huge networks of atoms all joined by strong covalent bonds — very high melting points. Examples:

• Diamond — each carbon bonded to four others; very hard; does not conduct electricity.
• Graphite — each carbon bonded to three others in layers that can slide (soft, slippery, good lubricant); has delocalised electrons so it conducts electricity.
• Graphene — a single layer of graphite; strong and conducting.
• Silicon dioxide (silica) — sand; very hard, high melting point.

Metallic bonding

Metal atoms are arranged in a giant structure where outer electrons are delocalised (free to move) — a "sea of electrons" surrounding positive metal ions, held by strong electrostatic attraction.

Properties of metals:

• High melting and boiling points — strong metallic bonds.
• Good conductors of electricity and heat — delocalised electrons carry charge and energy.
• Malleable (can be bent and shaped) — layers of ions can slide over each other.

Alloys

A pure metal is too soft for many uses because its layers slide easily. An alloy is a mixture of a metal with other elements. The different-sized atoms distort the layers, making it harder for them to slide, so alloys are harder than pure metals.

States and changes of state (energy)

When a substance melts or boils, energy is used to overcome the forces between particles, not to raise the temperature — this is why temperature stays constant during a change of state. Stronger forces → higher melting/boiling points.

Nanoparticles (overview)

Nanoparticles are very small particles (1–100 nm) with a very large surface area to volume ratio, which gives them different properties from the bulk material. They are used in catalysts, sun creams, electronics and medicine. There may be risks that are not yet fully understood.

Exam tips

• Match structure to properties — this is the heart of this unit. For any property (melting point, conductivity, hardness), explain it using the structure and bonding.
• Ionic compounds conduct only when molten or dissolved — explain why (ions free to move).
• Simple molecular substances have low melting points because the intermolecular forces are weak (not the covalent bonds).
• Explain metal properties and alloys using delocalised electrons and sliding layers.
• Be able to draw dot-and-cross diagrams for simple ionic and covalent substances.