What is Radioactivity

We have to start by first defining what an isotope is, before looking at radioactivity itself.
Isotopes are atoms of the same element - i.e. they have the same atomic number (the number of protons in their nucleus) - but they have a different mass - i.e. they have a different number of neutrons in their nucleus. Being an isotope does NOT mean something is radioactive. Having the same number of protons means they are the same element and undergo the same chemical reactions.

When the nucleus of an isotope is unstable, it undergoes radioactive decay by the spontaneous emmission of radiation. The stability of a nucleus depends on the number of sub-atomic particles it contains. It takes energy to hold these particles together, and if there is not enough energy to do this, the nucleus unstable.

Symbol used to indicate a substance is radioactive

Types of Radiation

There are three types of radiation emitted by a radioactive nucleus.

Alpha particles - these are identical to a helium nucleus, and consist of two protons and two neutrons. 42He2+ , where the 4 is the mass number, the two is the atomic number of number of protons, and the 2+ is the charge. You could write it as just He2+ but in some common questions you might get asked, it is important to remember the 4 and the 2.

Beta particles - these are high energy electrons, emitted from the nucleus.

Gamma rays - these are electromagnetic radiation, like light, radio waves, x-rays. They are often (but not always) emitted at the same time as alpha or beta particles, removing eccess energy from the nucleus.

When a radioactive nucleus emits an alpha particle, its mass is reduced by four units and the number of protons, and neutrons, is each reduced by two. Changing the number of protons means that the nucleus is now a different element.

When a nucleus emits a beta particle, what is happening is that a neutron is turning into a proton and an electron (the beta particle). so the mass is the same but the number of protons has increased by one and the number of neutrons has deceased by one. Again this means that the nucleous is now a different element.

Gamma rays do not change the mass, number of protons or neutrons in the nucleous. They just reduce the energy.

Many naturally occuring things are slightly radioactive - you, your chair, the ground.

Penetrating power of radiation

Alpha particles are emmited with high energy from the nucleus and can penetrate through several centimetres of air, and cause ionisation of molecules. At each ionisation event they loose some energy, and eventually slow up to the speed of normal gas molecules. A sheet of paper is enough to stop them, or a thin sheet of metal foil.

Standing next to an alpha emitor is not dangerous, as the particles will not reach you! The danger comes if you have eaten or breathed in something that gives off alpha particles, as then parts of you get ionised, and the ions can then undergo reactions.

Beta particles are high energy electrons, but don't have as much energy as alpha particles. However, being very very small compared to an alpha particle, they travel very much faster and have a greater range. They travel several metres through air, go straight through paper and it takes a few millimetres of aluminium foil to stop them. Being very small, they are less likely to pass close enough to ionise molecules.

Gamma rays are VERY much more penetrating. They can travel through many metres of air, through paper, and aluminium foil, and it needs lead plates, several centimetres thick, to stop them.

Half-Life of an Isotope

The half-life is the time it takes for half the nuclei of that type present at that moment in time to under go radioactive decay.

After one half-life there will be half the original number left. After two half-lifes there will be one quarter of the original number left, after three half-lifes there will be one eighth, and so on.

Half-lifes vary a great deal from a few seconds or less, to years, to millions of years. As those with long half-lifes will be radioactive for a very long time, disposing of them safely is a problem. Those with short half-lifes give out a lot of radioactivuty in a short time, so are dangerous to work with, but soon become inactive and safe to dispose of.

We hope this has helped you understand a bit more about radioactivity.
We will be adding more pages about radioactivity soon.

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