Elements are naturally unstable as they break down over time to more stable forms.

Radioactive elements

Explanation: Radioactivity is the phenomenon in which an unstable nuclei is converted to a stable nuclei by emitting radiations in the form of alpha decay, beta decay and gamma decay.

Alpha decay : When a larger nuclei decays into smaller nuclei by releasing alpha particle. In this process, the mass number and atomic number is reduced by 4 and 2 units respectively.

General representation of alpha decay :

Beta decay : In this process, a neutron is converted into a proton and an electron. In this decay, the atomic number is increased by 1 unit.

General representation of beta decay :

Gamma decay : In this process, the unstable nuclei transformed into stable nuclei by releasing gamma rays. In this process, the mass number remains same.

General representation of gamma decay :

Hence the correct answer is radioactive elements.

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