What materials that naturally break down by shooting away parts of their atoms?

Radioactive naturally breaks down by shooting away parts of their atoms

Explanation:

The isotopes which are not in balance like the neutrons and protons of those isotopes are not in balance with each other are known as radio active. But these radio active come back to the state of a normal isotope when the energy which is extra in them gets released and stabling down to a normal isotope.

But these radiations released can be harmful to the human body if they are of high level. These material of radio active occur normally like naturally but some times they are man made also.            

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