Question

When the nucleus is said to be unstable?

Answer 1

Nucleus of certain elements such as uranium, thorium, radium have a property of spontaneously emitting invisible, highly penetrating and high energy radiation. The unstable nucleus of radioactive substances undergoes spontaneous decay by self emission of either alpha $\left(\alpha \right)$ , beta $\left(\beta \right)$ or gamma $\left(\gamma \right)$ radiations to form a more stable nucleus.