I have heard that if n/p ratio of a substance becomes more than 1.56, it becomes radioactive. But for Cobalt-60, n/p= 1.22. Yet it is radioactive. Why is that so? Thanx in advance...
That is only a rule of thumb. Co60 is radioactive because it is energetically favorable for it to be so.
There is a simple rule named Mattauch rule with just 2 exemptions: Out of two isotopes with same mass and whose proton number differs by one, at least one must be radioactive. The 2 exemptions are: Antimony-123 and tellurium-123 are both stable Tantalum-180 and hafnium-180 are both stable (Also tungsten-180 decays by alpha decay, not electron capture as it should by Mattauch rule). A consequence of Mattauch rule is: By Mattauch rule, the only stable isotopes with odd number of protons and neutrons are the 4 light isotopes D, Li-6, B-10 and N-14. All others should be radioactive. Tantalum 180 is violating Mattauch rule and so is the 5th stable odd-odd isotope. There are still just these 5. Cobalt 60 is odd-odd, and is not one of these 5 stable isotopes, so it is radioactive.