Q. What do you mean by the term radioactivity? Why some elements are radioactive but some are not?
plz give me answer of each question in three lines​

breaking apart of atoms of certain elements to release rays of energy or particles.

Atoms are the building blocks of elements. However, some atoms have an excessive number of neutrons or protons in their nuclei. This causes an imbalance in the forces that keep them together, resulting in an excessive amount of internal energy. These atoms are referred to as radioactive or unstable.

hope this is helpful...