# How many neutrons can an atom have?

To date, the nucleus with the highest neutron/proton ratio is $\text{_12^40"Mg}$. Its n/p ratio is 28/12 = 2.33. It has a half-life of about 1 ms.
The heaviest known nucleus is $\text{_112^285"Cn}$. It has 173 neutrons. Its n/p ratio is 173/112 = 1.54, and its half-life is 29 s.