The big idea: Almost every molecule in a living thing is built around the element carbon. Carbon is special because one carbon atom can form four covalent bonds.
What four bonds allow
- Carbon atoms can link into long chains, branches and rings.
- So a small set of elements can be arranged into a huge variety of different molecules.
- This is the basis of all the macromolecules of life.
One carbon atom forms four covalent bonds, letting carbon skeletons grow into chains, branches and rings.
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- Carbon compound (organic molecule)
- A molecule built around a skeleton of carbon atoms — for example a carbohydrate, lipid, protein or nucleic acid.
- Covalent bond
- A bond in which two atoms share a pair of electrons.
- Macromolecule
- A very large molecule built from many smaller repeating subunits (for example a polysaccharide, a protein or a nucleic acid).
- Monomer
- A single small subunit that can be joined to others to build a larger molecule (for example glucose or an amino acid).
- Polymer
- A large molecule made of many monomers joined together (for example starch, a polymer of glucose).
Why four bonds matters: Because each carbon can bond four times, carbon skeletons can grow and branch in almost endless ways.
No other common element of life builds such varied frameworks — this single property is why carbon is the backbone of every biological molecule.
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The big idea: Carbon makes four covalent bonds, single or double, to carbon and to other elements. That lets it build chains, branches and rings.
The points to remember
- A covalent bond is a pair of electrons shared by two atoms.
- A carbon atom always makes four bonds in total.
- A bond can be single (one shared pair) or double (two pairs): a double bond counts as two.
- Carbon bonds to other carbon atoms and to other non-metals: hydrogen, oxygen, nitrogen, phosphorus, sulfur.
- So carbon skeletons can be unbranched or branched chains.
- They can also close into single rings or several rings joined together.
Remember it as: Four bonds, any partner, any shape.
Real example: olive oil: Most of the fat in olive oil is oleic acid: an unbranched chain of 18 carbon atoms, with one C=C double bond in the middle.
One trap when you list the properties:
Give separate properties: "Carbon is strong" or "carbon is stable" is not a property that explains variety. Name the bonds, the partners and the shapes.
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The big idea: Biologists measure tiny things, so they all use the same units and the same prefixes, agreed between countries.
The points to remember
- Scientists everywhere use the same units, the SI units, by international agreement.
- So a result measured in one country can be checked and compared in another.
- A prefix in front of a unit makes it bigger or smaller by a fixed power of ten.
- Cells are measured in micrometres (µm); molecules and viruses in nanometres (nm).
Remember it as: Kilo, centi, milli, micro, nano: each step down makes the unit smaller.
| Prefix | Symbol | Means | Example |
|---|---|---|---|
| kilo | k | × 1000 (10³) | 1 kg = 1000 g |
| centi | c | ÷ 100 (10⁻²) | 1 cm = 0.01 m |
| milli | m | ÷ 1000 (10⁻³) | 1 mm = 0.001 m |
| micro | µ | ÷ 1 000 000 (10⁻⁶) | 1 µm = 0.001 mm |
| nano | n | ÷ 1 000 000 000 (10⁻⁹) | 1 nm = 0.001 µm |
Real sizes: A red blood cell is about 7 µm across. A ribosome in a bacterium is about 20 nm across, more than 300 times smaller.
How this is tested: This comes up on Paper 2, as the first part of a long question. Each mark is one separate property of carbon.
What you may be asked
- Paper 2, Outline [4]: the chemical properties of carbon that let it form diverse compounds. Give separate, scoring points: four bonds, covalent bonds, single or double bonds, bonds to many elements, chains/branches/rings.
Exam-style question — outline carbon's bonding properties
Outline the chemical properties of carbon that allow it to form a wide variety of compounds. [4]
How to score all four marks
- Four covalent bonds. Each carbon atom can form four covalent bonds, so many other atoms can join to it.
- Bonds to itself and to other elements. Carbon bonds strongly to other carbon atoms and to elements such as hydrogen, oxygen and nitrogen, adding different chemical groups.
- Many shapes. Carbon skeletons can be straight chains, branched chains or rings, so the same atoms can be arranged in many different ways.
- Single or double bonds. Carbon can form single or double bonds, such as the C=O double bond in a carboxyl group. (One mark for each separate point, up to 4.)
Final answer
Carbon forms four covalent bonds; bonds to itself and to other elements (H, O, N); makes chains, branches and rings; and forms single or double bonds — together these build a huge variety of molecules.
Why this scores full marks: Each sentence is a separate, distinct property — four bonds, bonds to many elements, many shapes, single or double bonds.
An 'outline' worth 4 marks needs four scoring points, not one idea written four ways.
| Property of carbon | What it means | Why it gives diverse molecules |
|---|---|---|
| Forms four covalent bonds | Each carbon atom can bond to up to four other atoms | A huge number of different structures can be built around each carbon |
| Single or double bonds | A carbon can share one or two pairs of electrons with a neighbour | Different bonds give different shapes and groups, such as C=O |
| Bonds to many elements | Carbon bonds to H, O, N, P, S and to itself | Different elements add different chemical groups and properties |
| Chains, branches and rings | Carbon skeletons can be straight, branched or ring-shaped | The same atoms can be arranged into many different shapes |