Carbon

Carbon (C) is a non-metal that appears throughout the 0620 syllabus: in bonding and structure (allotropes), metal extraction (reducing agent), organic chemistry (backbone of all organic molecules), and environmental chemistry (CO2).

Where it appears in 0620

Carbon is one of the most cross-cutting elements on the course:

• Giant covalent structures: diamond and graphite are the structure-and-property comparison, with fullerenes as the simple-molecular contrast.
• Extraction of metals: carbon reduces the oxides of metals below it in the reactivity series.
• Organic chemistry: every organic molecule is built on a carbon skeleton.
• Air quality and climate change: CO2 as a greenhouse gas and CO as a toxic product of incomplete combustion.

Position in the periodic table

• Symbol: C
• Atomic number: 6
• Group 14 (IV), Period 2
• Electron configuration: 2, 4
• Forms 4 covalent bonds in most compounds

Allotropes

Carbon has several allotropes — different structural forms of the same element.

Diamond

• Giant covalent structure
• Each C bonded to 4 others in a rigid 3D tetrahedral lattice
• Very high melting point (strong covalent bonds throughout)
• Very hard (rigid lattice)
• Does not conduct electricity (no free electrons or ions)
• Insoluble in all solvents
• Uses: cutting tools, drill bits, jewellery

Graphite

• Giant covalent structure arranged in layers
• Each C bonded to 3 others in flat hexagonal layers
• One delocalised electron per carbon atom between the layers
• Very high melting point (strong covalent bonds within layers)
• Soft and slippery (weak forces between layers let them slide)
• Conducts electricity (delocalised electrons move along the layers)
• Uses: lubricant, pencil lead, electrodes (e.g. in electrolysis)

Fullerenes

• Molecules of carbon atoms in hollow shapes (e.g. C60, buckminsterfullerene)
• C60: 60 carbon atoms arranged like a football
• Each C bonded to 3 others
• Simple molecular, so a relatively low melting point (weak forces between the molecules)
• Uses: drug delivery, lubricants, catalysts, nanotechnology

Carbon as a reducing agent

Carbon (as coke) extracts metals that lie below it in the reactivity series:

Metal oxide	Equation
Iron(III) oxide	2Fe2O3(s) + 3C(s) -> 4Fe(l) + 3CO2(g)
Zinc oxide	ZnO(s) + C(s) -> Zn(l) + CO(g)
Lead(II) oxide	2PbO(s) + C(s) -> 2Pb(l) + CO2(g)

Carbon is the reducing agent: it removes oxygen from the metal oxide and is itself oxidised to CO or CO2. Metals above carbon (Na, Ca, Mg, Al) cannot be won this way and need electrolysis.

Carbon in organic chemistry

Carbon is the backbone of all organic compounds:

• Carbon atoms link into chains and rings
• Each carbon makes 4 bonds (single, double, or triple)
• Key homologous series: alkanes, alkenes, alcohols

Carbon compounds in environmental chemistry

Compound	Role
CO2	Greenhouse gas; product of complete combustion
CO	Toxic gas from incomplete combustion
CaCO3	Limestone, eroded by acid rain

See carbon across IGCSE chemistry for the cross-topic map.

Key facts at a glance

Fact	Detail
Symbol	C
Proton number	6
Group / Period	14 (IV) / 2
Bonds formed	4 covalent bonds
Allotropes	Diamond (hard, insulator), graphite (soft, conductor), fullerenes
Key role	Reducing agent for metals below it in the reactivity series

Common exam mistakes

• Saying graphite conducts “because it is a metal”. Graphite is a non-metal; it conducts because each atom bonds to only three others, leaving one delocalised electron free to move.
• Explaining hardness by bond strength. Diamond and graphite have the same strong C-C bonds. Graphite is soft because of the weak forces between layers, not weaker bonds.
• Calling fullerenes giant covalent. C60 is a simple molecule, so it has a low melting point compared with diamond and graphite.
• Using carbon to extract reactive metals. Carbon reduces only the oxides of metals below it (iron, zinc, lead). Aluminium and magnesium are above carbon and need electrolysis.

Exam-style questions

Graphite is used to make the electrodes in electrolysis cells. Give two properties of graphite that make it suitable, and explain each in terms of its structure. (4 marks)

Mark scheme

• graphite conducts electricity [1]
• because each carbon bonds to only three others, leaving delocalised electrons that move along the layers [1]
• graphite has a very high melting point / is chemically inert [1]
• because of the many strong covalent bonds, so it does not melt or react during electrolysis [1]

Examiner note: a property must be paired with a structural reason — “it conducts” on its own scores only half the available marks.

Compare the structures of diamond and graphite, and use them to explain why diamond is hard but graphite is soft. (4 marks)

Mark scheme

• in diamond each carbon is bonded to four others in a rigid 3D tetrahedral lattice [1]
• in graphite each carbon is bonded to three others in flat hexagonal layers [1]
• diamond is hard because the rigid network of strong covalent bonds resists any movement [1]
• graphite is soft because the layers are held by weak forces and slide over each other [1]

Examiner note: do not write that graphite’s bonds are weak — the covalent bonds are strong; it is the forces between layers that are weak.