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Atom, facts about atoms

Everything in the universe is made of incredibly tiny particles called atoms. Atoms are so small that 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 of two kinds of particles: protons (which are electrically positive) and neutrons, so-called because they are electrically neutral. The orbiting electrons are electrically negative.

Atom, fun facts about atoms
The orbit changes its position so rapidly that it seems to weave a solid shell round the nucleus	A hydrogen atom consists of a single electron moving round a single proton, like a planet in orbit round the Sun

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 atoms meaning 'that which cannot be divided'.

Atom, facts about atoms
Here the orbit is shown as a shell, but cut away to reveal the proton at the center	A carbon atom contains six protons and six neutrons in its nucleus. Six electrons circle the nucleus, two in an inner shell and four in an outer shell

More than 2,000 years ago the Greek philosopher Democritus suggested that if something was halved, then halved again, and again, then eventually there would be tiny pieces which could never be any smaller. Today, of course, we know that the atom is made of smaller particles.

Atoms are so incredibly small that if the key was enlarged until it stretched 25,000 miles around the Earth each atom in the key would still be no larger than this red ball

Ninety-two different atoms (and therefore elements) exist in nature. Others have been made artificially in the laboratory. Scientists list atoms 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 (atomic force). In a solid this atomic forces is strong. In a liquid it is weak. In a gas the atomic forces is very weak and 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.

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