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What are Radioactive Nuclei?

What is in the Nucleus of an Atom?

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.
See further information for a more detailed description of an atom.

What are Radioactive Isotopes?

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 some examples).

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