Carbon: tetravalency and catenation
Carbon (2, 4) can neither gain nor lose four electrons easily, so it shares them, forming four covalent bonds. It also links to itself in long chains: catenation.
Carbon is everywhere
Carbon is found in the Sun, the planets and the earth's air. It is in carbonate rocks such as limestone, marble and dolomite, and in coal, petroleum and natural gas. Every living thing contains carbon compounds such as carbohydrates, proteins and fats. In the human body, carbon is the second most common element after oxygen. The soot from a kerosene lamp and the black remains of burnt wood or paper are mostly carbon.
Why carbon shares electrons
Carbon has 4 electrons in its outer shell. To complete its octet, it would have to gain 4 electrons (to become like neon) or lose 4 (to become like helium). Both need far too much energy, so carbon does not form ions. Instead it shares its four electrons with other atoms and forms four covalent bonds. This is tetravalency (tetra means four). In methane, CH₄, carbon shares one pair of electrons with each of four hydrogen atoms.
How to draw methane
- Electron dot structure: write C in the centre with its 4 electrons as crosses. Put an H on each side with 1 dot, so each C-H pair sits between the atoms.
- Check: carbon has 8 electrons around it; each hydrogen has 2.
- Line structure: draw C in the centre with four single lines to four H atoms. Put them above, below, left and right.
- Label: carbon atom (2, 4), hydrogen atom, shared pair, covalent bond.
Chains, branches and rings (catenation)
| Shape | What it looks like | Example |
|---|---|---|
| Straight chain | Carbon atoms in a row: C-C-C-C | Butane, C₄H₁₀ |
| Branched chain | A carbon atom joined to the side of the chain | Isobutane, also C₄H₁₀ |
| Ring | Carbon atoms joined in a closed loop | Benzene, C₆H₆ |
Why carbon forms so many compounds
- Catenation: the strong C-C bond lets carbon form long, stable chains, branches and rings.
- Tetravalency: each carbon has four bonds to share with carbon, hydrogen, oxygen, nitrogen, sulphur or halogens.
- Multiple bonds: carbon can also form double and triple bonds with carbon, oxygen or nitrogen.
- Isomers: compounds with the same formula but different structures add even more compounds.
- Carbon forms more compounds than all the other elements put together. Their study is organic chemistry.
The rice, dal and ghee on your plate are all carbon compounds. Archaeologists use carbon-14 dating to find the age of old wood, bones and cloth dug up at ancient sites.
2 marks: “Carbon has the configuration 2, 4. Explain why it forms covalent bonds rather than C⁴⁺ or C⁴⁻ ions.” To complete its octet, carbon would have to gain or lose 4 electrons. Both need far too much energy, so it shares its 4 electrons and forms 4 covalent bonds.
1 marks: “What is catenation?” The ability of carbon atoms to bond with one another to form long chains, branches or rings.
2 marks: “Give two reasons why carbon forms a very large number of compounds.” Catenation: strong C-C bonds allow long chains, branches and rings. Tetravalency: four bonds to many kinds of atoms. (Multiple bonds and isomerism also add to the number.)
Carbon: four bonds to share, and chains to spare (tetravalency and catenation).