Properties of Materials
Linking Structure to Properties
The physical properties of a substance — its melting point, boiling point, hardness and electrical conductivity — depend on its type of bonding and structure. Understanding this link is one of the most important ideas in chemistry: if you know the bonding, you can predict the properties. The exam frequently tests your ability to explain properties in terms of structure and bonding, so it is essential to link every property back to the forces involved.
Ionic Compounds
Ionic compounds such as sodium chloride have giant ionic lattice structures. They have high melting and boiling points because a large amount of energy is needed to break the many strong electrostatic forces between the ions. They do not conduct electricity when solid because the ions are held in fixed positions. However, they do conduct when molten or dissolved in water because the ions become free to move and carry charge. Ionic compounds are often brittle — when a force is applied, layers of ions shift and like charges line up, causing repulsion that shatters the crystal. Most ionic compounds are also soluble in water because water molecules can surround and separate the individual ions.
Ionic compounds: high melting points (strong ionic bonds), conduct electricity only when molten or dissolved (ions must be free to move), do not conduct as solids (ions fixed in place).
Simple Molecular Substances
Simple molecular substances (such as water, carbon dioxide and oxygen) have strong covalent bonds within each molecule but weak intermolecular forces between molecules. It is these weak intermolecular forces that are overcome when the substance melts or boils — not the covalent bonds. This means simple molecular substances have low melting and boiling points and are often gases or liquids at room temperature. They do not conduct electricity in any state because they have no free electrons or ions to carry charge. It is a very common exam error to say the covalent bonds are weak — the bonds within the molecules are strong; it is the forces between molecules that are weak.
Larger molecules have stronger intermolecular forces because there are more electrons that can create temporary dipoles. This means larger molecules tend to have higher boiling points. For example, the boiling point of methane (\(CH_4\), small molecule) is much lower than that of octane (\(C_8H_{18}\), larger molecule). The strength of the intermolecular forces depends on the size of the molecule, not the strength of the covalent bonds within it.
Giant Covalent Structures
Giant covalent structures contain billions of atoms all bonded by strong covalent bonds in a continuous network. They have very high melting points because many strong covalent bonds must be broken. Carbon has several giant covalent allotropes — different structural forms of the same element, each with distinct physical properties:
Carbon can form four covalent bonds. In diamond, each carbon atom bonds to four others in a rigid tetrahedral arrangement — making it extremely hard and unable to conduct electricity (no free electrons). In graphite, each carbon bonds to three others in flat hexagonal layers with weak forces between layers, allowing them to slide over each other (making graphite soft and slippery). One electron per carbon atom is delocalised within each layer, so graphite does conduct electricity. Fullerenes (e.g. \(C_{60}\) buckminsterfullerene) are hollow carbon molecules shaped like balls or tubes. Graphene is a single layer of graphite — one atom thick, incredibly strong, an excellent electrical conductor, and a potential material for electronics and composites.
Carbon can form four covalent bonds. Diamond (4 bonds per C, very hard, non-conductor), Graphite (3 bonds per C, layers slide, conducts electricity), Graphene (single layer of graphite, strong, conducts), Fullerenes (hollow molecules, e.g. \(C_{60}\)).
Polymers and Metals
Polymers are very long chain molecules made of repeating units. They are held together by strong covalent bonds along the chain and weaker intermolecular forces between chains. Their properties depend on what the repeating unit is and how the chains are arranged. Metals have high melting points, conduct electricity (delocalised electrons), and are malleable (layers of positive ions can slide over each other without breaking the metallic bond, because the delocalised electrons can adjust to the new positions). Metals are also good conductors of heat because the delocalised electrons transfer kinetic energy quickly through the structure.
You can use data such as melting point and boiling point to predict the state of a substance at a given temperature. If the temperature is below the melting point, the substance is solid. Between melting and boiling point, it is liquid. Above the boiling point, it is a gas. For example, water (melting point 0°C, boiling point 100°C) is solid below 0°C, liquid between 0°C and 100°C, and gas above 100°C. This skill is commonly tested using data tables.