What is a radioactive is isotope?

Answer 1

an isotope that is radioactive

Answer 2

to make it simple on you the difference is an isotope is stable. a a radioactive isotope is not stable and usually has a half life.
An isotope is a particular kind of atom ... Like Carbon-12 6protons and 6 neutrons and Carbon-14 6protons and 8neutrons.

The extra neutrons sometimes makes that particular isotope. Carbon-14 is radioactive. But for larger atoms all of their isotopes are radioactive -- like uranium.

The isotopes 'decay' into a different substance by 3 methods ... they can emit alpha partices, or beta particles, or pure energy called gamma rays.
Phosphorous-31 is quiet element. It is not radio active

If suppose one neutron is added then it becomes an isotope P-32. This is radio active

Same way right from P-31 if one neutron is taken out then P-30 would result. This is also radio active. These isotopes are said to be radio active isotopes.