Carbon builds millions of different compounds, more than any other element. Its four hands are half of the answer. The other half is how strongly those hands hold on.
A carbon atom has 4 outer electrons and shares 4 more, so it makes 4 bonds. It can use them on other carbon atoms, in chains, branches and rings, and on hydrogen, oxygen and nitrogen. It can also put two or three of its hands on the same partner, making a double or a triple bond.
How strong is a bond?
Chemists measure a bond's strength as the energy it takes to pull the two atoms apart. One bond on its own takes far too little energy to measure, so bond energies are given for a huge, fixed number of bonds: 602,000 billion billion of them, about as many as there are carbon atoms in 12 grams of carbon.
| Bond | Energy |
|---|---|
| C–H | 413 kJ |
| C–C | 346 kJ |
| C–O | 358 kJ |
| C–N | 305 kJ |
| C=C | 614 kJ |
| C≡C | 839 kJ |
Carbon's single bonds to carbon, hydrogen, oxygen and nitrogen are all strong, and all close in strength. A chain of carbon atoms holds together about as firmly as the hydrogens round it, and an -OH or -NH₂ group sits on the chain as firmly as the chain itself. So one molecule can carry rings, chains and groups side by side, and all of them last.
Drag the model to turn it. Pinch to zoom, or use the buttons.
Strong, and slow to react
Petrol, wood and paper all burn in oxygen, and burning gives out energy. At room temperature they still sit in the air for years, because burning starts only once something carries the molecules over the energy hill, such as a flame or a spark. Water leaves carbon chains alone too: candle wax, a mix of long carbon chains, can lie in water for years and come out unchanged.