Key Stage 4 science · Chemistry – Structure, bonding and the properties of matter
GCSE carbon structures: diamond, graphite and graphene for parents
Carbon is the element behind life, fuels and plastics, and also behind some of the strangest materials in modern science. GCSE chemistry asks why one element can form millions of compounds, and why diamond, graphite, fullerenes and graphene, all made only of carbon atoms, behave so differently. Students answer by linking each property to the arrangement of atoms and bonds.
Carbon chemistry and carbon structures, explained
A carbon atom has four electrons in its outer shell, so it forms four covalent bonds. It can bond strongly to other carbon atoms and to atoms such as hydrogen, oxygen and nitrogen, and it can join to other carbon atoms in long chains, branched chains and rings. This is why carbon is the basis of a vast number of organic compounds, both natural, such as sugars, proteins and the hydrocarbons in crude oil, and synthetic, such as plastics, medicines and fibres. Compounds with similar structures and similar chemical behaviour can be grouped into families, which chemists call homologous series. Courses usually start with the alkanes, which have only single bonds between carbon atoms, with methane, ethane, propane and butane as the first members, and show how each differs from the next by one carbon atom and two hydrogen atoms.
Diamond and graphite are both made of carbon atoms joined by strong covalent bonds, but in different arrangements. In diamond each carbon atom is bonded to four others in a rigid three-dimensional lattice. There are no free electrons or ions, and a great deal of energy is needed to break so many strong bonds. So diamond is extremely hard, has a very high melting point, and does not conduct electricity. That is why it is used in cutting and drilling tools and in jewellery.
In graphite each carbon atom is bonded to three others, forming flat layers of hexagons. The fourth outer electron from each atom is delocalised and can move along the layer, so graphite conducts electricity. The layers are held together only by weak intermolecular forces, so they can slide over each other. This makes graphite soft and slippery, which is why it is used as a lubricant, in pencils and as electrodes. It still has a high melting point, because the bonds inside each layer are strong.
Graphene is a single layer of graphite, one atom thick. It is very strong for its mass, conducts electricity well and is almost transparent, so scientists are researching its use in electronics and composite materials. Fullerenes are molecules of carbon shaped like hollow balls or tubes, such as the 60-atom buckminsterfullerene and nanotubes. Nanotubes are very strong and conduct electricity, and fullerenes are being investigated for drug delivery, catalysts and lubricants. Some of these uses are still in development. The key GCSE skill is a short chain of reasoning: structure, then bonding, then property, then use.
Words your child will hear
- Organic compound
- A compound based on carbon atoms, usually bonded to hydrogen and often to other elements.
- Hydrocarbon
- A compound made only of hydrogen and carbon atoms, such as methane.
- Allotropes
- Different structural forms of the same element, such as diamond and graphite.
- Giant covalent structure
- A huge network of atoms joined by strong covalent bonds, as in diamond.
- Fullerene
- A molecule of carbon atoms forming a hollow shape such as a ball or a tube.
- Graphene
- A single layer of graphite, one atom thick, made of carbon atoms in hexagons.
- Delocalised electron
- An electron that is free to move through a structure instead of being held between two atoms.
What the National Curriculum says
The programme of study for Key Stage 4 says:
- Students should be taught about the bonding of carbon leading to the vast array of natural and synthetic organic compounds that occur due to the ability of carbon to form families of similar compounds, chains and rings
- Students should be taught about structures, bonding and properties of diamond, graphite, fullerenes and graphene
Objectives from the National Curriculum in England, © Crown copyright, used under the Open Government Licence v3.0.
Try it: a model answer on diamond and graphite
Diamond and graphite are both forms of carbon. Explain why graphite conducts electricity and is soft, but diamond does not conduct and is very hard.
- Start with what is the same. Both are giant covalent structures made only of carbon atoms joined by strong covalent bonds. The difference is the way the atoms are arranged.
- Describe diamond. Each carbon atom forms four covalent bonds to four other atoms in a rigid three-dimensional lattice. All four outer electrons are used in bonding.
- Link diamond’s structure to its properties. With no free electrons or ions, there are no charge carriers, so it does not conduct. Many strong bonds must be broken to scratch it, so it is very hard.
- Describe graphite. Each carbon atom forms only three covalent bonds, which makes flat layers of hexagons. One outer electron per atom is left over and is delocalised.
- Link graphite’s structure to its properties. The delocalised electrons can move along the layers, so graphite conducts. The layers are held together by weak intermolecular forces, so they slide easily, which makes graphite soft.
Graphite has layers with delocalised electrons that conduct and weak forces between layers that let them slide. Diamond has every atom held by four strong bonds in a rigid lattice, so it is hard and has no free electrons.
Common mistakes and how to help
Thinking diamond and graphite are different elements
They look and behave completely differently, so it seems they must be different substances. Both contain only carbon atoms. They are allotropes, with the same atoms joined in different patterns.
What helps: Ask, as a thought experiment, what would be made if each one were burned completely in plenty of oxygen. Both give only carbon dioxide, which shows they contain only carbon.
Believing that pencil lead is lead, or that graphite conducts because it is a metal
The name “lead” survives from history, and graphite conducts, which is a property students associate with metals. Pencils contain graphite mixed with clay. Graphite is a non-metal that conducts because of delocalised electrons.
What helps: Ask what a metal-like property needs. The answer is not being a metal, it is having electrons that are free to move. This is a good example of explaining a property with structure.
Saying that graphite is soft because its bonds are weak
Students learn that layers slide and mix up bonds with forces. Inside each layer the covalent bonds are strong, which is why graphite has a high melting point. It is the forces between the layers that are weak.
What helps: Ask which bonds or forces are broken when the layers slide, and which stay intact. A good answer names both, and uses the word “between” carefully.
Assuming that all carbon compounds come from living things, or that carbon can form any number of bonds
The word “organic” suggests living things, and carbon’s versatility can suggest it has no limits. In fact carbon forms four bonds in the compounds students meet, and many carbon compounds are made in factories.
What helps: Pick a plastic bottle and a sugar cube, and ask what they have in common. Then draw methane and check that the carbon atom has exactly four bonds.
How to help at home
- Smudge some graphite. Rub a soft pencil firmly on paper, then smear the mark with a finger. Layers of graphite rub off easily. Ask what that tells you about the forces between layers, and why a hard pencil leaves a lighter mark.
- Build 3D models. Join marshmallows or balls of modelling clay with straws or lolly sticks. Make a four-bond carbon with its four neighbours, then flat hexagons. Compare how rigid each is, then link that to hard diamond and slippery graphite.
- Hunt for carbon uses. Look around the home for things made from carbon materials: pencils, plastics, fuel, a carbon-fibre bike or tennis racket. Ask what property makes each material right for its job.
- Draw the family tree of hydrocarbons. Draw methane, ethane and propane as carbon and hydrogen atoms with lines for bonds. Count the bonds on each carbon atom and notice that each is always four. Then predict the drawing for butane before you check it.
Related topics
Builds on
- Key Stage 3: States of matter and the particle model
More on chemistry – Structure, bonding and the properties of matter
- Key Stage 4: Bonding and properties of matter
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