There are four fundamental forces that you should be aware of:

• gravity
• electromagnetic force
• strong nuclear force
• weak nuclear force

The protons located in the nucleus all possess a positive charge. This means that they all repel one another, an example of electromagnetic force. Based on this you might expect the protons to push one another away in the nucleus. However, this is not the case because of another force which keeps the nucleus together: strong nuclear force.

The strong nuclear force

The strong nuclear force is responsible for holding together the neutrons and protons (or nucleons) within the nucleus. Meson and baryons, collectively known hadrons, consist of the strong nuclear force whereas leptons do not.

The strong nuclear force is able to act over only a very short range and can be either repulsive or attractive:

• short range attraction between adjacent nucleons is up to a distance of approximately 3 x 10-15m or 3 femtometers
• short range repulsion between adjacent nucleons is up to a distance of approximately 0.5 x 10-15m or 0.5 femtometers

Stable and unstable nuclei

When bismuth consists of a proton number of 83 it contains a stable nuclei at the highest number of protons. At a proton number higher than 83 all nuclei are unstable and are said to be radioactive.

Radioactive elements emit:

• alpha particles (?): they consist of two protons and two neutrons (helium nucleus)
• beta particles (?): this is a high speed electron
• gamma rays (?): this is a photon

Alpha decay (?)

Alpha decay is when the radioactive parent nuclide decays into a daughter nuclide by emitting an alpha particle. For example, thorium-232 decays into radium-228 by emitting an alpha particle.

It is important that on both sides of a decay equation both the proton and nucleon numbers balance. Therefore, if nuclide X decays into nuclide Y, the nucleon number of Y has to be lower than that of X and the proton number of Y has to be two less than that of X:

Beta decay

Beta decay is when the neutron in a parent nuclide decays into a proton by emitting a beta particle and an anti-neutrino. For example, carbon-14 decaying into nitrogen-14.

When this occurs, there is no change in the nucleon number but the proton number decreases by one in Y:

An anti-neutrino is the anti-particle of a neutrino. This particle does not have a charge and a mass of close to zero

Gamma radiation

A photon of electromagnetic radiation emitted from a nuclide is known as a gamma ray.