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Isotopes are variations of the same element with the same atomic number but a different atomic mass. Isotopes of an element have the same number of protons but a different number of neutrons - which is why their atomic mass is different.
On the Periodic Table, the relative atomic mass is featured which is a weighted average from all the abundant isotopes of an element.
The isotopes of some elements occur naturally and are said to be 'stable'; others are unstable, meaning that they only exist in a state of decay.