CBSE Class 12-science Answered
Nuclei like to maintain a specific ratio of neutrons-to-protons. If this ratio is not achieved, then the system will experience some level of instability. Generally, the first twenty elements prefer a ratio of one-to-one. Elements beyond the first twenty will prefer more neutrons than protons. Consequently, the ratio will be a number that is greater than one. The preferred value for that ratio is not well-defined.
Elements with atomic number more than 82 have neutron to proton ratio of more than one and hence get unstable.