Nuclear instability causes radioactivity

The stability of an atomic nucleus is determined by the balance between two opposing forces:

• Strong nuclear force - This force acts to hold the protons and neutrons within the nucleus together.
• Electrostatic force - This is the force that causes positively charged protons to repel each other.

A nucleus may become unstable and thus radioactive if it has:

• Too many neutrons
• Too few neutrons
• A total number of nucleons (protons and neutrons) that is too high
• Insufficient binding energy to hold it together

This instability leads to radioactive decay, during which the nucleus emits energy in the form of radiation until it reaches a stable configuration.

The four main types of nuclear radiation

The radiation emitted by unstable nuclei can be categorised into four main types, each with distinct characteristics:

These types of emissions are capable of ionising atoms, which means they can displace electrons from atoms or molecules, transforming them into ions.

Key radiation properties

Nuclear radiation types differ in their ionising power, speed of emission, and how far they can travel through different materials:

Gamma rays, in particular, possess short wavelengths and high energies, enabling them to penetrate more deeply than alpha or beta particles.

When does alpha decay occur?

Alpha decay is characteristic of very heavy nuclei, such as uranium, which have more nucleons than can be stably maintained:

For instance:

$^{238}_{92}\text{U} \rightarrow {}^{234}_{90}\text{Th} + {}^{4}_{2}\text{He}$

In this process, the emission of an alpha particle decreases both the atomic number and mass number of the original nucleus, leading to a more stable configuration.

When does beta-minus emission occur?

Beta-minus decay typically happens in isotopes with an excess of neutrons.

This process converts a neutron into a proton while emitting an electron:

${}_{75}^{188}\text{Re}\to {}_{76}^{188}\text{Os}+{}_{-1}^0\text{e}+{\bar{\nu }}_e$

This transformation increases the atomic number by one, but the mass number remains unchanged, resulting in a different element.

Note: You might sometimes see the electron (${}^{0}_{-1}\text{e}$) represented as a beta-minus particle (${}^{0}_{-1}\beta^{-}$) instead, but it means the same thing.

When does beta-plus emission occur?

Beta-plus decay typically happens in isotopes with an excess of protons.

This process converts a proton into a neutron while emitting a positron (beta-plus particle):

^1^3_\phantom{1}_7\text{N}\rightarrow^1^3_\phantom{1}_6\text{C}+^0_+_1\beta^++\nu_\text{e} ParseError: Double superscript at position 3: ^1^̲3_\phantom{1}_7…

This transformation decreases the atomic number by one, but the mass number remains unchanged, resulting in a different element.