Isotopes that decay with a nuclear break-up and emit a significant amount of energy are said to be

Answer to this is Radioactive isotopes.

Isotopes are the species of the same element having different atomic masses that means the number of protons remains the same but number of neutrons do differ. For example and are the two isotopes of Hydrogen ( ).

Radioactive isotopes are the isotopes which release some kind of energy in the form of alpha particles, beta particles or gamma radiation. Examples of each of the decay processes are :

Alpha Decay: In this decay one alpha particle having atomic mass 4 and atomic number 2 or we can say a He molecule will come out.

Beta Decay: In this decay a particle is emitted increasing the atomic number of the reactant by 1 unit.

Gamma Radiation: In this type of reaction only radiation is emitted out which does not change the original molecule.

a compound is a molecule made of atoms from different elements.

a. by the combination of an unlimited number of elements