Define the term Radioactive? When do the elements become radioactive ?

There are two kinds of atoms....

Those which are radioactive, and those which are not. The atoms which are not radioactive are stable, their nuclei are at a low energy and do not undergo decay to get to a lower energy state or a stable state. Those which are radioactive have nuclei which are higher in energy and shed energy by undergoing decay (alpha or beta decay) in order to get to a lower energy state or to a stable state. An atom which is stable cannot suddenly become radioactive without some outside intervention, like absorbing a neutron, proton or cosmic ray.

For example....

6C14 --> -1e0 + 7N14 ......... -1e0 = electron = beta(-) particle ... beta decay

6C12 --> no decay .............. stable isotope

C-14 is radioactive and unstable. The isotope undergoes beta decay to form a stable atom of nitrogen. C-12 is stable and does not undergo decay.

Every atom is an isotope of one element or another. There are no atoms which are not isotopes. Some isotopes are stable and do not undergo decay. Radioactive isotopes are unstable and decay be beta or alpha emission to form an isotope of a different element.

Like other have said, elements don't normally become radioactive, they either are or aren't. We do know that elements with atomic number than 83 are all radioactive

An element or isotope is radioactive if the atoms spontaneously decay by emitting alpha or beta particles or a gamma ray and changing into a different element or isotope.

elements / isotipes do not "become" radioactive. They are naturally radioactive or naturally stable.

An element can be changed into a radioactive element or isotope when bombarded by particles or high energy photos. But without that bombardment the element is stable.