What shape do molecules take?

That time, that place

By the mid-nineteenth century benzene had chemists stuck. Six carbons and six hydrogens — but carbon has four bonds and hydrogen one.

Lay them in a chain and bonds are left over. No arrangement balanced.

Why this question

Kekulé's answer was a ring. Join the six carbons in a circle rather than a line and the spare bonds at each end meet.

He later said the idea came while dozing by the fire, from an image of a snake biting its own tail. Whether that is true is disputed — the story first appears in a commemorative speech twenty-five years afterwards.

What was found

True or not, the ring changed chemistry. The idea that a molecule is a figure with a shape takes hold here.

Until then a formula said what was present and how much. Now it said how things were arranged — and why substances made of identical atoms can behave differently.

The synthetic dye industry starts here. So does the notion of designing a drug: arranging a molecule to produce a property you want.