ATOM

Everything in the universe is made of incredibly tiny particles called atoms. Atoms are so small that even if ten thousand million could be placed end to end they would measure half an inch. An atom is like a solar system in miniature. It has a central 'sun' or nucleus which has a number of 'planets' or electrons 'orbiting' around it.

The nucleus itself is made up of two kinds of particles: protons (which are electrically positive) and neutrons, so called because they are electronically neutral. The orbiting electrons are electrically negative.

Different elements are made of different atoms, the difference being in the number of orbiting electrons (called the atomic number) and the number of protons plus neutrons in the nucleus (called the mass number).

The word 'atom' comes from the Greek atomos meaning 'that which cannot be divided'. More than 2,000 years ago the Greek philosopher Democritus suggested that if something was halved, then halved again, and again, then eventually there will be tiny pieces which could never be any smaller. Today, of course, we know that the atom is made of smaller particles.

Ninety-two different atoms (and therefore elements) exist in nature. Others have been made artificially in the laboratory. Scientists list atom in order of atomic number in the periodic table.

Like our solar system, atoms are mostly space. In 1919, Lord Rutherford bombarded gold foil with particles emitted from a piece of radium and found that most of them passed straight through. Rutherford also concluded that practically all of the atom's mass is concentrated in its nucleus.

In an element, atoms are held together by mutual attraction. In a solid, this attraction is strong. In a liquid it is weak. In a gas the atoms move freely. Heating can weaken the attraction between atoms, thus changing an element from a solid to a liquid, and then to a gas.