Question

I radioactivity is limited to heavy elements​

Answer 1

Radioactivity starts after Bi(83) and the elements after it have a bad ratio of proton and neutron. Basically the element which has same no. Of protons and neutrons are most stable. But in radioactive elements neutron's number is much greater then proton. It mean they have a bad ratio of p/n which make them unstable. So they decay their nucleus to become stable

Answer 2

The lighter elements do not show any radioactivity because the nuclei of lighter elements contain nearly equal number of protons and neutrons. On the other hand, the nuclei of heavier elements (elements having higher atomic numbers), contain more neutrons than protons.