Lucy Dalton, of Ilam School, asks :-

What are atoms made of?

Murray Matthews, a nuclear chemist with the Department of Health's National Radiation Laboratory in Christchurch, responded.

Atoms are made of energy contained in particles called protons, neutrons and electrons. An atom can be imagined as a miniature Solar System with a nucleus (Sun) at its centre, with electrons (planets) orbiting it.

The nucleus makes up almost all the weight of an atom, and contains the protons and neutrons. Protons have a positive electric charge while neutrons are of similar size to protons but have no electric charge. Each electron has a negative electric charge and is about a millionth the size of a proton.

The number of electrons in an atom always equals the number of protons in its nucleus, so the atom is electrically neutral, although atoms can lose or gain electrons in chemical reactions. The number of neutrons in the nucleus is usually similar to the number of protons although in some atoms there can be either too many or not enough neutrons, and this causes the nucleus to be unstable or radioactive.

The number of protons in an atom is known as its atomic number and this determines the chemical nature of all the elements. Every element has its own number, from 1 for hydrogen the lightest of all the elements to 92 for uranium, the heaviest of the elements which occur naturally.

What protons, neutrons and electrons are made of is one of the big puzzles physicists are working on today.

It is interesting that, like the Solar System, most of the volume of an atom contains nothing at all.