SC7 · Types of substance · Topic 1 — Key concepts in chemistry | GCSE Edexcel Separate Sciences

Revise the key ideas

Classify substances by structure and bonding: ionic lattices, simple molecules, giant covalent networks and metallic lattices. The pattern of bonds and mobile charge carriers explains their properties.
Ionic substances have strong attractions between ions and usually high melting points; they conduct when molten or dissolved because ions move, but not as solids.
Simple molecular substances generally have low melting and boiling points because intermolecular attractions are weak. They usually lack mobile charge carriers and conduct poorly.
Giant covalent structures have a large network of strong covalent bonds. Breaking these bonds needs a lot of energy, so these substances have high melting points. They are not made of separate small molecules.
Most metals are shiny, solid, fairly dense and good electrical conductors, with high melting points. These are trends, not absolutes: mercury is liquid at room temperature and alkali metals have relatively low melting points.
Use several measured properties to infer structure. Appearance or solubility alone rarely identifies a bonding type reliably.

Allotropes are different structural forms of the same element. Diamond and graphite are both pure carbon but have very different properties.
In diamond each carbon atom forms four strong covalent bonds to other carbons in a three-dimensional giant network.
Local unit only, not an isolated molecule or a flat structure.
The strong rigid network makes diamond very hard and gives it a high melting or sublimation temperature under appropriate conditions. It is useful in cutting tools and drill tips.
Diamond does not normally conduct electricity because its outer electrons are involved in bonds and there are no mobile delocalised electrons or ions.
Diamond is insoluble in water: dissolving it would require disrupting a giant bonded network, not just separating small molecules.
A diagram with a central carbon and four neighbours shows a local unit. The bonds continue through the material; diamond is not a five-atom molecule.

In graphite each carbon atom forms three covalent bonds in flat hexagonal layers. Strong bonds within the layers give a high melting or sublimation temperature.
Attractions between layers are relatively weak, so layers can slide. This makes graphite soft and useful as a lubricant and in pencil cores.
Layer sections are schematic; bonds extend beyond the drawing.
In graphite, one outer electron from each carbon atom is delocalised: it is free to move through the layers rather than belonging to one bond. These electrons carry charge, so graphite conducts electricity and can be used for electrodes.
Do not explain graphite conduction by mobile ions or sliding layers. The charge carriers are delocalised electrons.
Graphene is a single layer of carbon atoms in a hexagonal network, one atom thick. It is strong, light and conducts electricity because of delocalised electrons.
A small section of a continuous layer; edges are not the bulk bonding pattern.
Graphene differs from graphite in the number of layers, not in the element. A rolled graphene-like sheet forms the structure of a carbon nanotube.

Fullerenes have hollow carbon cages or tubes. Buckminsterfullerene, C₆₀, is a molecule of sixty carbon atoms in a cage containing pentagonal and hexagonal rings.
Different structures of the same element explain different properties.
The covalent bonds within a C₆₀ molecule are strong, but attractions between separate molecules are weaker. Molecular fullerene solids differ from diamond's continuous covalent network.
Hollow cages can enclose other substances; their structures are investigated for uses such as delivering substances. Use potential applications carefully rather than assuming every application is routine.
Carbon nanotubes are long, hollow carbon structures that are very strong and have useful electrical properties. They can strengthen other materials and, because of their large surface area, provide a support for catalysts.
A ball-and-stick cage model exaggerates gaps and bond thickness. Its purpose is to show connectivity and shape, not literal atom sizes.

A metal can be modelled as a regular lattice of positive metal ions surrounded by delocalised electrons from outer shells.
Metallic bonding is the strong electrostatic attraction between positive metal ions and delocalised electrons. These electrons can move throughout the metal rather than belonging to one atom.
Delocalised electrons move through the metal and carry electric charge, so metals conduct as solids and liquids. Electrons and lattice vibrations also transfer thermal energy.
Electrostatic attraction to mobile electrons holds the metal together.
Layers of metal ions can slide while attraction to the delocalised electrons remains. Metals are malleable, meaning they can be hammered into shape, and ductile, meaning drawn into wires.
Layer sliding can occur without the brittle charge alignment of an ionic crystal.
Many metals have high melting points because strong metallic attractions need much energy to overcome. Melting-point and conductivity comparisons depend on the particular metal, not a universal one-factor rule.
Alloys mix a metal with other elements. Different-sized atoms can disrupt sliding of layers, often making an alloy harder than a pure metal. Alloy behaviour depends on its composition.

Dot-and-cross diagrams show the electrons involved in bonding, but do not show the real three-dimensional shape. Ball-and-stick models show how atoms join and are arranged, using sticks to represent bonds.
A flat lattice or layer diagram omits part of a three-dimensional structure. Space-filling models show relative occupied space but can hide bonding details.
The metal model explains mobile electrons but static pictures omit particle motion. Real solid ions vibrate rather than remaining completely motionless.
Explain a use by connecting structure to property: graphite has mobile electrons, so it can be an electrode; diamond has four bonds per carbon in a rigid network, so it can cut hard materials.
When comparing unknown materials, consider melting point, conduction in different states and solubility. Check exceptions such as conducting graphite before assigning a bonding type.