What does radioactive mean???

Answer 1

It means particles are periodically emitted from unstable nuclei, these particles can cause photograph negatives to expose.

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Wiki Radioactivity

Answer 2

A radioactive element one whose nucleus decomposes spontaneously by emitting alpha rays (helium nucleus), beta rays (electrons), or gamma rays (electromagnetic radiation). The element changes into another element if alpha or beta rays are emitted. There are other more esoteric transformations but this will cover the ground floor.