Atom, molecule, ion: one family, four names

About 2,400 years ago, Democritus imagined cutting matter again and again until a piece could not be cut any more. He called it atomos, meaning indivisible. Nice idea, but nobody did a single experiment to check it.

In 1808 an English school teacher, John Dalton, used atoms to explain two real laws: conservation of matter and definite proportions. The irony? The indivisible atom is divisible after all. Electrons, protons and neutrons are its fundamental particles.

Can we at least see one? An ordinary microscope cannot show anything smaller than about 500 nm. An atom is about 0.2 nm across. An electron microscope uses electrons, whose wavelength is much shorter than light's, so it can picture layers of carbon atoms in graphite.