Where the idea of atoms came from

Your pen, your phone, the air touching your face: everything, living or not, is made of atoms. Think of two ladders:

• a body is made of organs, then tissues, then cells, then proteins... then atoms;
• a house is made of rooms, then walls, then bricks, then silicates... then atoms.

"If you keep breaking something into smaller and smaller pieces, what is the last piece?" More than 2,000 years ago, Acharya Kanada in India thought about this. He said that if you divide matter (dravya) again and again, you finally reach tiny pieces that cannot be divided. He called them *parmanus*. His ideas are recorded in the Sanskrit Vaisesika Sutras.

Kanada said parmanus join in twos (dyads) and threes (triads) to build everything in the universe, though he did not say in what proportions.

In Greece, Leucippus and Democritus had a similar idea. They called the smallest pieces *atomos*, Greek for "cannot be cut". Note: all of these were ideas, imagined by careful thinking, not tested by experiments.

In 1808, John Dalton put forward his atomic theory, based on experiments: all matter is made of indivisible atoms. This was the first scientific description of matter.

But a good theory raises new questions:

• What are atoms made of?
• What would they look like?
• What makes the atoms of one element different from another's?

This chapter follows the scientists who answered them.