Why does carbon form so many molecules?

A champion of chemistry

There are millions of known organic molecules, whilst there are only 118 chemical elements in total. This diversity can be explained by the unique properties of the carbon atom.

Four bonds, not one more

The carbon atom has 4 valence electrons. To comply with the octet rule (8 electrons surrounding it), it forms exactly 4 covalent bonds, whether single, double or triple. This is known as the tetravalence of carbon.

This tetravalence enables it to bond:

• with hydrogen (H) atoms;
• with oxygen (O), nitrogen (N) and chlorine (Cl) atoms, amongst others;
• and, most importantly, to other carbon atoms, thereby forming chains.

The secret: highly stable carbon–carbon bonds

Unlike silicon, which is also tetravalent, carbon forms very stable C–C bonds, even in long chains. It is this stability that enables the existence of giant molecules such as DNA or proteins, all of which are built on a carbon backbone.

Representing a molecule

An organic molecule is represented by its molecular formula (a simple count of atoms) or its structural formula (showing the bonds). For example, ethanol:

Formule brute : C2H6O
Formule semi-developpee : CH3-CH2-OH

Common mistake

Don’t assume that carbon always forms four bonds with four different neighbours: what matters is that the total number of bonds formed is four, whether they are single, double or triple bonds. In carbon dioxide (O=C=O), the carbon has only 2 neighbours, but 4 bonds in total (2 double bonds).