Here's why:

* Atomic Structure: Atoms are made up of protons, neutrons, and electrons. The nucleus, containing protons and neutrons, determines the element's identity.

* Stability: The balance between protons and neutrons in the nucleus determines an atom's stability.

* Radioactive Decay: Unstable atoms have an imbalance in their nucleus and attempt to achieve stability by releasing energy and particles. This process is called radioactive decay.

Key points:

* Not all elements are radioactive: Most elements found naturally are stable.

* Radioactive isotopes: Atoms of the same element can have different numbers of neutrons (these are called isotopes). Some isotopes are radioactive, while others are not.

Examples of radioactive elements:

* Uranium (U)

* Plutonium (Pu)

* Carbon-14 (an isotope of Carbon)

* Iodine-131 (an isotope of Iodine)