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  • Created by: Sophie
  • Created on: 06-04-14 21:08


All substances are made of atoms.
These have electrons on the outside and a nucleus in the middle.
The nucleus consists of neutrons and protons and is very small (the majority is empty space).

The atomic number of an atom is the number of protons in its nucleus.
The atomic mass is the number of protons + neutrons in its nucleus. 

NB: Isotopes of an atom have the same number of protons, but different numbers of neutrons.

In some types of atom, the nucleus is unstable and will decay into a more stable atom. This radioactive decay is completely random. You can't say when any unstable nuclei is going to decay. 

Inside any nucleus, the protons and neutrons are held together by the incredibly powerful strong nuclear force, which overcomes the electrostatic repulsion between the protons.
A balance exists between these two forces.
This means that certain numbers of protons and neutrons can make a stable nucleus, whilst others groups will be more or less unstable. These will eventually decay to produce a more stable arrangement.

As the atomic number (no. of protons) increases, atoms seem to need more neutrons. 
Once we get to more than 82 protons, the nuclei are no longer stable and we're into radioactive elements.
Elements with more than 92 protons are so unstable that they don't exist naturally and have to be made by us in nuclear reactors. 

Radioactive decay gives out heat.

You can heat the substance up, subject it to high pressure or magnetic fields, and it will not affect the rate of decay at all.

There are three types of particles that can be given out when an unstable nucleus decays.

  • alpha particles ( ( )
  • beta particles ((
  • gamma rays ( ( 

Anything that gives off ( will give off ( and/or ( particles too.

After an ( has been emitted from the nucleus, the atomic number of the atom will have changed. Therefore the atom will have changed into a different element.

Alpha Particles are Helium Nuclei

Alpha particles (( are made up of 2 protons and 2 neutrons. An alpha particle is the same thing as the nucleus of a helium atom. So it can be written as (  or  ( 

When a nucleus emits an alpha particle, it loses 2 protons so its atomic number decreases by two, and it loses 2 protons and 2 neutrons so its mass number decreases by 4.

Example:   an alpha-decay equation (

Alpha decay occurs in heavy elements for example uranium. These heavy elements contain too many protons to be stable, and become more stable by emitting an alpha particle.

Because of their big size, they are also slow moving, so they do not penetrate far into materials, in fact they can be blocked by paper, skin, or a few centimetres of air.
However it also means that they are strongly ionising, meaning they bash into a lot of atoms and knock electrons off them before they slow down which creates…


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