The number of sigma and pi bonds in a benzene molecule are, respectively:

Benzene (C6H6) is the simplest aromatic hydrocarbon. Its structure was proposed by the German chemist August Kekulé in 1865. He described it as a ring of six carbon atoms with alternating single and double bonds, and he is said to have imagined it in a dream of a snake biting its own tail.

To count the bonds:

• Every single bond is one sigma (σ) bond.
• Every double bond has one sigma bond and one pi (π) bond.

In benzene, there are 6 carbon-carbon bonds in the ring and 6 carbon-hydrogen bonds. Each of these 12 bonds contains one sigma bond, giving 12 sigma bonds. Three of the carbon-carbon bonds are double bonds, and each of these adds one pi bond, giving 3 pi bonds.

In fact, the pi electrons in benzene are not fixed between particular pairs of carbon atoms. They are delocalised, spread evenly over the whole ring. This is called resonance. Because of this, all six carbon-carbon bonds in benzene are of equal length (about 139 picometres), between the length of a single and a double bond. This is why benzene is often drawn as a hexagon with a circle inside. The delocalisation makes benzene very stable, so it prefers substitution reactions (such as nitration and halogenation) over addition reactions.

Each carbon atom in benzene is sp2 hybridised, and the molecule is flat (planar), with bond angles of 120°. Benzene was first isolated by Michael Faraday in 1825. It is used to make plastics, dyes, detergents and drugs, but it is toxic and a known carcinogen.

Why not the other options?

• ✗6 and 6 is wrong because the C-H bonds also count as sigma bonds.
• ✗9 and 3 counts only the ring bonds and the double bonds incorrectly.
• ✗12 and 6 counts too many pi bonds.