Question

What is radioactivity?​

Answer 1

The emission of elementary particles by some atoms when their unstable nuclei disintegrate

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Answer 2

$<b>In other atoms, especially heavy ones such as those of uranium, this energy is insufficient, and the nuclei are unstable. An unstable nucleus spontaneously emits particles and energy in a process known as radioactive decay. The term radioactivity refers to the particles emitted.</b>$