Joanna Sem, of Ilam School, asks :-

What colour is an atom?

John Campbell, a physicist at the University of Canterbury, responded.

What a tricky question. There are several possible answers.

We see things when light is reflected off them into our eyes. Atoms are so small that white light waves dont register the atom. Therefore in that case an atom is not coloured.

A metal is just a large collection of atoms and all metals are coloured. Gold and silver have given their names to the colour they display. However almost all metals are silvery because the colour has more to do with the electrons that conduct electricity through the metal than the type of atoms present.

Atoms can emit their own light. For example, if we excite sodium atoms in an electrical discharge they glow with yellow light. These are sometimes used as street lamps. Similarly neon atoms produce the red light in neon advertising signs. Each lamp contains millions upon millions upon millions of atoms so we still are not dealing with an individual atom.

About five years ago a very clever friend of mine, then working at IBM Research Labs in California, managed to trap a single atom all by itself. He used a laser to excite the isolated atom which then glowed with its characteristic colour. The colour of an atom depends on how the electrons which orbit the atom are arranged around the nucleus of the atom. So in that sense atoms are indeed coloured but only when they are excited by an electrical discharge or some other means such as a powerful laser.