What is radioactive decay?

Isotopes of elements (especially large elements) are often unstable - this means that they have an excess of neutrons or mass in their nucleus. To help them become more stable, the nucleus will naturally emit subatomic particles from the nucleus to lose mass. This is called radioactive decay and it is totally random. 

There are 4 main types of radioactive decay, and you need to know the differences between them in terms of their structure, penetration power, and how ionising they are. This 3-minute read aims to provide you with the quick revision you need on the differences between these radiations, including decay equations.

‍Alpha decay:

This is where an alpha particle (α) is emitted from the nucleus. An alpha particle consists of 2 protons and 2 neutrons. It has a relative atomic mass of 4, and an atomic number of 2. Alpha particles are slow and heavy (in comparison to other radiations), which means they are easily stopped by a few centimetres of air, a piece of paper, or the skin. Because of their size and their +2 charge, they are highly ionising. Alpha decay results in the mass number decreasing by 4, and the atomic number decreasing by 2, as seen in the example below:

Beta decay:

This is where a beta particle (β) is emitted from the nucleus, after a neutron changes into a proton in the nucleus of an unstable atom. It is effectively an electron, with negligible mass and a -1 charge. Beta particles are much faster than alpha particles and more penetrating. They can pass through skin, air, and paper and are only stopped by thin aluminium. They have a much lower ionsing power than alpha particles. Beta decay does not change the mass number of the radioactive atom but does increase the atomic number by 1, as seen in the example below:

Gamma decay:

This is the release of excess energy, usually as a result of electrons dropping down to a lower energy level, in the form of electromagnetic gamma waves (γ). Gamma waves travel at the speed of light, have no mass and no charge and are weakly ionising. They have the highest penetration power, and are only stopped by thick lead or concrete (although even then some will pass through). Because they have zero mass and zero charge they do not change either the atomic mass or the atomic number of the radioactive atom.

Neutron emission:

This is the release of single neutrons from the nucleus of the atom, decreasing the atomic mass by 1 (no change to the atomic number). This type of radiation only really occurs naturally in the upper atmosphere, through the absorption of cosmic rays. Neutron emission occurs artificially through the process of nuclear fission, where a large nucleus absorbs a high-speed neutron, and is split into 2 smaller nuclei, releasing more neutrons and thermal energy.

This handy diagram helps to summarise the differences in the penetrating power of the three radiations discussed above.

This table should help to summarise the key differences between the 3 main types of natural radioactive decay: