Stable and Unstable Nuclei

Protons and neutrons (known as nucleons) are found inside the nucleus.

Protons are positively charged, therefore experience electrostatic repulsion. Neutrons are neutral, hence do not experience any electrostatic force.

Nucleons have masses therefore are gravitationally attracted to one another. However, the gravitational force between nucleons is much smaller than the electrostatic repulsion.

In order to keep the nucleus together, a third force is needed: the strong nuclear force.

The strong nuclear force holds quarks together and since both neutrons and protons contain quarks, the strong force works equally between all nucleons to keep the nucleus together.

The strong nuclear force varies depending on distance \left(1 \: \text{fm} = 1×10^{-15} \: \text{m}\right):

• Less than 0.5 \: \text{fm}: The strong nuclear force is repulsive to ensure the nucleus is not crushed.
• Between 0.5 \: \text{fm} and 3 \: \text{fm}: The strong nuclear force is attractive and larger than the electrostatic repulsion.
• After 3 \: \text{fm}: The strong nuclear force quickly falls to zero and has no effect.

The maximum value of attraction for the strong nuclear force occurs around 1 \: \text{fm}, which is the average separation between nucleons in a nucleus (nuclear separation).

The strong nuclear force is also known as the strong interaction.

We can look at how the nuclear separation between nucleons effects the strong nuclear force between these nucleons.

Note how when the nuclear separation is at around 1 \: \text{fm} in the graph, the strong nuclear force between the nucleons is at a maximum attraction.

Alpha decay involves the nucleus (the parent nucleus) releasing an alpha particle and decaying into a smaller nucleus, known as a daughter nucleus.

An alpha particle has 2 protons and 2 neutrons, the same as the nucleus of a Helium atom. The daughter nucleus has 2 less protons and 4 less nucleons compared to the parent nucleus. This results in a new element.

The general nuclear equation for alpha decay is:

\textcolor{f21cc2}{ _Z^A X \rightarrow _{Z-2}^{A-4}Y + _2^4\alpha}

As an example, Uranium-238 undergoes alpha decay to produce Thorium-234 and an alpha particle:

^{238}_{92} U \rightarrow ^{234}_{90} Th +  _2^4\alpha

Neutrinos were initially hypothesised to ensure that energy was conserved during a beta decay. It was first thought that electrons were the only particle emitted during beta decay. However it was found that the total energy after the decay was less than the total energy before the decay. To solve this problem, Wolfgang Pauli suggested another particle also carried energy away, which had no charge (to ensure conservation of charge) and no/negligible mass (as it has not been detected). The neutrino was subsequently observed, affirming Pauli’s original hypothesis.