Which best describes a radioactive isotope?

(a) an atom is unstable.
(b)an atom is stable.
(c)there is only one other isotope.
(d)there are many isotopes

Option (A) is the correct answer.

Explanation:

An isotope with an unstable nucleus which decomposes readily and results in the emission of radiation and a nuclear electron or helium nucleus achieves a stable nuclear composition is known as a radioactive isotope.

For example,

Here isotope of polonium decomposes in order to achieve a stable nuclear composition.

Thus, we can conclude that an atom is unstable best describes a radioactive isotope.

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