Why c - 14 is radio active though n/p ratio is less than 1.5?
There is no "magic rule"; see the article on "nuclear drip line": http://en.wikipedia.org/wiki/Nuclear_drip_line
That is not a rule. Most radioactive elements do not have n/p > 1.5. That's a myth
Do not confuse the statement "There are no stable nuclei with n/p > 1.5" with the statement "All unstable nuclei have n/p > 1.5". See the difference? The first one is true. The second one most definitely isn't.
Lead-207 is (experimentally1) stable with 82 protons and 125 neutrons, 125/82=1.524.
There are a few other examples, see the linked list.
That's exactly my point. This rule is just an approximate rule of thumb...
I've studied in class about something called 'magic numbers' where nuclei having 2, 8, 20, 28, 50, 82 or 126 protons or neutrons or both (seperately) are exceptionally stable compared to their respective neighbouring nuclides. So, they too make an exception.
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