What is it?

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. You can heat the substance up, or subject it to high pressure or strong magnetic fields - in fact, do whatever you like to it - and you won't affect the rate of decay in the slightest. When an unstable nucleus decays, there are three ways that it can do so.  It may give out:- an alpha particle (we use the symbol a) a beta particle (symbol b) a gamma ray (symbol g) Many radioactive substances emit a particles and b particles as well as g rays. In fact, you won't find a pure g source; anything that gives off g rays will also give off a and/or b too.

ALPHA PARTICLES

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) 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

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. 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

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

Alpha particles are easy to stop, gamma rays are hard to stop. Particles that ionise other atoms strongly have a low penetrating power, because they lose energy each time they ionise an atom. Radioactive decay is not affected by external conditions. You need to know the information in this table:-

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.

Type of Radiation	Alpha Particle	Beta Particle	Gamma Ray
Symbol
Mass	4	1/2000	0
Charge	+2	-1	0
Speed	slow	fast	very fast (speed of light)
Ionising Ability	high	medium	0
Penetrating Power	low	medium	high
Stopped by	paper	aluminum	lead

We talk about "radioactive isotopes" - but what's an isotope? For a start, just because something is called an isotope doesn't necessarily mean it's radioactive. You can think of different isotopes of an atom being different "versions" of that atom. Consider a carbon atom. It has 6 protons and 6 neutrons - we call it "carbon-12" because it has an atomic mass of 12 (6 plus 6). If we add a neutron, it's still a carbon atom, but it's a different isotope of carbon. One useful isotope of carbon is "carbon-14", which has 6 protons and 8 neutrons. This is the atom we look for when we're carbon dating an object. So isotopes of an atom have the same number of protons, but a different number of neutrons.