Certain atoms are radioactive because in their atoms

In the periodic table the elements which have atomic number Z > 92 their nuclides have highly unstable.  So, that they emit  $\alpha ,\beta \text{\hspace{0.17em}and }\gamma$ rays by its own. This is called radioactivity. Generally actinide series (5f-black) elements exhibit radioactivity. The neutron to proton ratio (n/p) should be greater than 1.