The phenomenon of spontaneous nuclear disintegration of radioactive elemets. Isotopes by emitting alpha, beta or gamma -rays in order to give stable nucleus is known as radioactivity. It is totally a nuclear phenomenon i.e., only depends upon the nucleons . It is alos known as activity and is given by A=lambdaN where lambda =decay constant N=no. of atoms at time t Activity per gram of the sample is known as specific activity . The cause of radioactivtiy of a nucleide can be attributed to a certain degree to its neutron to proton ratio i.e., n/p ratio. Lighter nucleides with n/p =1 or slightly greater than one, are non-radioactive or stable nucleides. Nucleides with Z gt 20 , requires large number of neutrons than protons to moderate the effect of the increasing protonic repulsions . Nucleides with Z gt 83 , the protonic repulsions are too large to overcome by proton -neutron interactions . The stable nuclei lie within zone of stability , all the nuclei which fall outside of this zone are invariably radioactive and unstable in nature. Nuclei which are above the stability zone have excess protons. These nuclei attain stability by making adjustments in n/p ratio for moderate value. Choose the correct statement(s)