Types of substance: Carbon structures and simple polymers — Types of substance — Edexcel Combined Science 1SC0

Explain the classification of elements and compounds into ionic, simple molecular (covalent), giant covalent, and metallic substances.
Explain how the structure and bonding of these substances dictate their physical properties, including relative melting and boiling points, solubility in water, and electrical conductivity.
Explain the properties of ionic compounds, specifically their high melting and boiling points due to strong forces between ions, and their electrical conductivity behaviour as solids, liquids, and aqueous solutions.
Explain the properties of typical simple molecular covalent compounds, including their low melting and boiling points due to weak intermolecular forces, and their poor electrical conductivity.

One element, different structures

Diamond and graphite both contain only carbon atoms, yet diamond is extremely hard while graphite is soft and slippery. They are allotropes: different structural forms of the same element. Their different properties come from how their atoms are arranged and bonded, not from containing different elements.

A covalent bond is a shared pair of electrons between atoms. Diamond and graphite are both giant covalent substances: very many carbon atoms are linked together by strong covalent bonds in an extended network. They are not made of separate small molecules.

Four vertically stacked panels compare carbon bonding arrangements. In diamond, a highlighted carbon has four tetrahedrally arranged neighbours, and bonds continue into a three-dimensional network. Graphite has three stacked hexagonal sheets: each highlighted carbon has three neighbours within its sheet. Solid lines represent strong covalent bonds; dashed lines between sheets represent weak forces. Arrows show sheets sliding and delocalised electrons moving along a sheet. Graphene is one such sheet, one atom thick, with electrons able to move across it. C60 is shown as a pentagon-and-hexagon cage, with its rear surface omitted for clarity. Each complete hollow molecule contains 60 carbon atoms. Two smaller, separate cages have weak forces between them, not covalent connections. All structures are representative and not to scale. — Carbon's properties depend on its bonding arrangement: a three-dimensional network, stacked layers, a single sheet or separate hollow cages. Representative structures, not to scale.

Diamond: a rigid three-dimensional network

In diamond, each carbon atom forms four covalent bonds with four other carbon atoms. These neighbours are arranged tetrahedrally around it, and the bonding continues throughout a three-dimensional lattice.

The strong bonds hold the atoms firmly in position throughout the structure. This makes diamond extremely hard, so diamond-tipped cutting tools can cut materials such as bricks and concrete. Its usefulness depends on its resistance to being scratched or worn away.

Diamond also has a very high melting point because a large amount of energy is needed to break its many strong covalent bonds. It does not conduct electricity: all four outer electrons of each carbon atom are involved in bonding, so there are no freely moving electrons to carry charge.

Graphite: strong layers that can slide

In graphite, each carbon atom forms three covalent bonds with other carbon atoms. These bonds produce flat layers of linked hexagons. The covalent bonds within each layer are strong, but the forces between neighbouring layers are weak.

The layers can therefore slide over one another easily. This makes graphite soft and slippery, allowing it to act as a lubricant: a material that reduces friction between moving surfaces. Sliding the layers does not require breaking the strong covalent bonds within them.

Each carbon atom also contributes one electron that is delocalised rather than held in a particular bond. These electrons can move along the layers and carry charge. Graphite therefore conducts electricity and can be used to make electrodes, which carry current into or out of an electrolyte during electrolysis.

Graphite has a very high melting point despite being soft. Melting requires overcoming its strong covalent bonding, whereas its softness comes from the weak forces between layers. These properties depend on different parts of the structure.

C₆₀: carbon arranged into hollow molecules

Fullerenes are forms of carbon with hollow cage-like or tube-like molecules. Buckminsterfullerene, C₆₀, contains 60 carbon atoms arranged in a hollow, roughly spherical cage. Its pentagons and hexagons give it a football-like shape, with each carbon bonded to three others.

Unlike diamond and graphite, C₆₀ consists of separate molecules. There are strong covalent bonds within each cage but weak intermolecular forces between cages. The rounded molecules can move past one another, making them useful as lubricants.

Although C₆₀ has some delocalised electrons within its molecules, these cannot move freely throughout the bulk material as they can through graphite's layers. C₆₀ therefore conducts electricity poorly.

The hollow cage can enclose other substances, giving fullerenes potential uses as carriers in drug delivery. Their surface also provides places for catalyst particles to attach, making them useful as catalyst supports.

Diamond and graphite

Both are carbon allotropes and giant covalent substances with very high melting points.

Substance	Structure and bonding	Property and use
Diamond	Each carbon bonds to four others in a tetrahedral, three-dimensional network	Very hard → cutting tools; no mobile electrons → does not conduct electricity
Graphite	Each carbon bonds to three others in hexagonal layers; weak forces between layers	Layers slide → lubricant; delocalised electrons carry charge → electrodes

C₆₀ and graphene

C₆₀: separate hollow molecules of 60 carbon atoms, arranged in pentagons and hexagons. Weak forces between rounded molecules allow movement → lubricant. Hollow cages → potential drug carriers. Poor electrical conductor.
Graphene: one atom-thick hexagonal carbon sheet. Strong covalent bonds → strong; very thin → lightweight; mobile delocalised electrons → electrical conductor.

Simple polymers

Large molecules containing long chains of carbon atoms.
Poly(ethene): covalently bonded carbon backbone with attached hydrogen atoms.
Separate long-chain molecules, not a diamond-like giant covalent lattice.

Link structure → property → use: graphite has mobile delocalised electrons, so it conducts electricity and can be used for electrodes.

For graphite, distinguish strong covalent bonds within each layer from weak forces between layers. The layers slide without breaking the covalent bonds.

Diamond is hard, meaning it resists scratching and cutting; this does not mean it cannot shatter.

Do not describe C₆₀ as a giant covalent lattice: it consists of separate molecules. A poly(ethene) chain is also a molecule, although a very large one.

One of two or more different structural forms of the same element.

A bond formed when atoms share a pair of electrons.

A continuous network of very many atoms joined by strong covalent bonds, rather than separate small molecules.

An electron that is not confined to one atom or one covalent bond and can move through part of a structure.

A form of carbon whose molecules have hollow cage-like or tube-like structures.

A single layer of carbon atoms, one atom thick, with each atom covalently bonded to three others in a hexagonal network.

A very large molecule made from many smaller units joined together by covalent bonds.

A substance that reduces friction between surfaces moving past one another.