Radioactive decay

The nuclei of some isotopes are unstable. They can split up or ‘decay’ and release radiation. Such isotopes are called radioactive isotopes or radioisotopes. When a radioactive isotope decays, it forms a different atom with a different number of protons.

Changing elements

When an atom emits alpha or beta radiation, its nucleus changes. It becomes the nucleus of a different element. This is because the number of protons in the nucleus determines which element the atom belongs to. These are the changes that occur to the number of particles in an unstable nucleus when it emits a radioactive particle:

Changes to particles

In each case, a different element is left behind.

Example 1

Uranium-230 nuclei emit alpha radiation and become nuclei of thorium-226:

Remember that an alpha particle is identical to a helium nucleus. Notice that:

• the mass number goes down by 4
• the atomic number goes down by 2

Example 2

Hydrogen-3 nuclei emit beta radiation and become nuclei of helium-3:

Notice that:

• the mass number stays the same
• the atomic number goes up by 1

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