what is known as radioactive elements.what is the reason for this.why is tritirium is radioactive while protium and dueterium is nor

Radioactive elements are those elements which emmits harmful radiations lke alpha,beta or gamma rays and this pehenomenon of emission of harmful radiations is called radioactivity.It was discovered by Henri Baquerel.Elements like uranium,Thorium etc. are radioactive elements.
Radioactivity arises because of unstability of such nucleus,and decay of nucleus leads to emission of radiations.The unstability of nucleus is due to either excess of neutrons or protons.To become stable nucleus undergo series of disintegration untill an stable nulceus is formed.
Tritium,protium and deuterium all are isotopes of hydrogen,but only tritium is radioactive and this is due to its unstable nucleus as it is more massive than its decay products.It has more no. of neutrons(2) and 1proton this combination of nucleons makes tritium unstable.This is not with the nulcei of protium and deuterium.