Question

A Radioactive Isotope is

	1.	Time for half of a sample to decay from parent isotope to daughter isotope for radioactive atoms
	2.	The technique of getting the age of something by using unstable isotopes
	3.	An element that has unstable nuclei and will transform itself into another element
	4.	The process of an element transforming into another element by changes of its nucleus

Answer 1

A radioactive atom or isotope has excess nuclear energy which makes this atom unstable and this radioactive isotope releases excess energy in the form of radiation and results in formation of some other nuclei.

Example ,

An unstable Carbon-14 nucleus emits beta radiation and transform to Nitrogen-14

Answer -- An element that has unstable nuclei and will transform itself into another element.