How to Tell if an element is radioactive

[dionysian]

My teacher mentioned that if an element has a neutron to proton ratio of 1.2 or higher than the element is radioactive. He also said there is on exception to this when the N/P ratio is lower than 1.2, but i didnt understand him. I cannot find this one exception in my book or on the internet. Can someone please help me here?

 

[@/@]

Search in a physics book over modern physics

Maybe these links can help you

http://en.wikipedia.org/wiki/Atomic_nucleus
http://en.wikipedia.org/wiki/Radioactive_series

 

[Gokul43201]

There are many synthetic isotopes with n/p < 1.2 that are beta emitters (eg: Cl-36, P-32). I don't know of any naturally occurring radionuclides with n/p < 1.2

Perhaps the folks over a Nuclear Engineering will have a better idea?

 

[dionysian]

Yes, Yes that was it. I believe he said that if the isotope is synthetic it is radioactive. But, doesn't that meen all synthetic isotopes are radioactive. And how can i tell if a certain isotope is synthetic...? do you just have to know them or is there a trick.

Pardon my lack of chemistry knowledge, I am just starting.