All substance are made of atoms.
These have electrons (e) around the outside,
and a nucleus in the middle.

The nucleus consists of
protons (p) and neutrons (n),
and is extremely small.
(Atoms are almost entirely made of empty space!)

In some types of atom, the nucleus is unstable, and will decay into a more stable atom. This radioactive decay is completely spontaneous.

It's not the same as what happens in a nuclear power station (where neutrons whizz around and hit uranium nuclei, causing them to split).

This form of Lithium is NOT radioactive - it's just an example of a simple atom.
Most radioactive substances have many more particles in their nucleus.

Many radioactive substances emit particles and particles as well as rays.

In fact, you won't find a pure source: anything that gives off rays will also give off and/or too.

Alpha particles are made of 2 protons and 2 neutrons.

This means that they have a charge of +2, and a mass of 4
(the mass is measured in "atomic mass units", where each proton & neutron=1)
We can write them as , or, because they're the same as a helium nucleus, .

Alpha particles are relatively slow and heavy.

They have a low penetrating power - you can stop them with just a sheet of paper.

Because they have a large charge, alpha particles ionise other atoms strongly.

Beta particles have a charge of minus 1, and a mass of about 1/2000th of a proton. This means that beta particles are the same as an electron.
We can write them as or, because they're the same as an electron, .

They are fast, and light.

Beta particles have a medium penetrating power - they are stopped by a sheet of aluminium or plastics such as perspex.

Beta particles ionise atoms that they pass, but not as strongly as alpha particles do.

Gamma rays are waves, not particles.
This means that they have no mass and no charge. So we sometimes write .

Gamma rays have a high penetrating power - it takes a thick sheet of metal such as lead, or concrete to reduce them significantly.

Gamma rays do not directly ionise other atoms, although they may cause atoms to emit other particles which will then cause ionisation.

We don't find pure gamma sources - gamma rays are emitted alongside alpha or beta particles. Strictly speaking, gamma emission isn't 'radioactive decay' because it doesn't change the state of the nucleus, it just carries away some energy.