Radioactivity

Radioactive Nuclei.

Links in pink will take you to the GCSE Chemistry site.

The nucleus of an atom contains protons and neutrons.
Protons are all positively charged and repel each other.
The protons in the nucleus are held together
by a force called the strong nuclear force (or strong interaction).

Protons and neutrons are sometimes called nucleons.
The number of protons is called the atomic number.
The number of protons plus neutrons is called the mass number.
The mass number is sometimes called the nucleon number.
For a detailed description of an atom, see the GCSE Chemistry site.

Isotopes of an element can exist
where the atoms have different numbers of neutrons.
A stable atom will have a stable number of neutrons.
For small atoms (containing less than 20 protons) a stable atom
will have approximately the same number of protons and neutrons.
An atom which is very different from this will be unstable.

The unstable nucleus will decay and emit radioactivity.
Radioactive nuclei are also called radioactive isotopes, radioisotopes
and radionuclides (they all mean the same thing).
After the decay, the nucleus will have changed into a more stable form.
The atomic number will have changed and a new element is formed
(see examples).

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