P6.1 Why are some materials radioactive?P6.1 Why are some materials radioactive?



Atoms consist of a nucleus and orbiting electrons. The nucleus of an atom contains protons and neutrons. It makes up most of the mass of the atom, but takes up virtually no space.

The electrons that orbit the atom are really small. They whizz around the outside of the atom. Their paths take up a lot of space, giving the atom its overall size (though it’s mostly empty space).

Every atom of a particular element has the same number of protons in its nucleus. However the number of neutrons isn’t fixed. Many elements have a few different isotopes – atoms with the same number of protons but different numbers of neutrons.

Radioactive Atoms

Some elements emit ionising radiation all the time – these elements are radioactive. Radioactive atoms are unstable as they break up (decay) to make themselves more stable. Unstable atoms can decay at random meaning you can’t predict when it will happen – it is completely unaffected by physical conditions (temperature) or chemical processes (bonding).

When an atom does decay, it spits out one or more of three types of ionising radiation – alpha, beta and gamma. During the decay process, the atom often changes into a new element.

Ionising radiation can transfer enough energy to break an atom or molecule into bits called ions – this is called ionisation. These ions can then go on to take part in other chemical reactions.


There are 3 types of ionising radiation (alpha, beta and gamma) which are all emitted by radioactive materials.

Alpha Radiation

Alpha particles are relatively big and heavy and fairly slow moving, meaning that they don’t penetrate far into materials as they’re stopped quickly. They are realised by a heavy nuclei (uranium).

An alpha particle is a helium nucleus (He) which is made up of 2 protons and 2 neutrons. Alpha particles have a mass of 4 and a charge of +2. Their decay always changes the element of the atoms that’s decaying, since it loses protons.




A typical alpha emission:

Beta Radiation

Beta particles move quite fast and they are quite small. They penetrate moderately into materials before they are stopped. Beta particles are released by nuclei that have too many neutrons.

During beta decay, a neutron in the nucleus turns into a proton, so the element changes, and a beta particle is emitted.

A beta particle is identical to an electron, with virtually no mass and a charge of -1.

A typical beta emission:

Gamma Radiation

After spitting out an alpha or beta particle, the nucleus might need to get rid of some extra energy. It does this by emitting a gamma ray – a type of electromagnetic wave which has no mass. They can penetrate a long way to materials without being stopped. Since a gamma ray is just energy, it doesn’t change the element of the nucleus that emits it.

Alpha particles are blocked by paper.

Beta particles are blocked by thin aluminium.

Gamma rays are blocked by thick lead.

Skin will stop alpha, a thin sheet of any metal will stop beta, and very thick concrete will stop gamma.


Half-life is the time taken for half of the radioactive nuclei now present to decay.

Radioactivity is measured in becquerels (bq) or counts per minute (cpm). 1 bq is 1 decay per second.

Each time an unstable nucleus decays and emits radiation that means one more radioactive nucleus isn’t there to decay later. As more unstable nuclei decay, the radioactivity of the source as a whole decreases – so the older a radioactive source is, the less radiation it emits.

How quickly the activity decreases varies a lot. For some isotopes it takes just a few seconds before nearly all the unstable nuclei have decayed. For others it can take millions of years. The problem with trying to measure this is that the activity never reaches zero, which is why we have to use the idea of half-life to measure how quickly the activity decreases.

Short half-life: the activity falls quickly, because lots of the nuclei decay in a short time.

Long half-life: the activity falls more slowly because most of the nuclei don’t decay for a long time