Why extra neutrons make the nucleus unstable?

[skepticwulf]

I understand that having neutrons in nuclei creates additional strong nuclear force which brings protons together, overcoming EM force thus forming different atoms but why extra neutrons bring instability? Wouldn't more strong force mean extra "glue" to hold nuclei together? yet it seems N/P ratio is best at 1/1, why??

 

[Simon Bridge]

There are several ways large numbers of neutrons lead to instabilities - i.e. the extra neutrons could cause the shape of the nucleus into an unstable configuration so small disturbances make it, basically, shake itself to bits.

You realize that neutrons are, themselves, unstable? So think about why they are apparently stable while inside a nucleus.

 

[skepticwulf]

Thank you.
I understand by your reply as well as by similar discussions.

 

[mfb]

The Pauli exclusion principle is relevant here. Neutrons are fermions - no two neutrons can occupy the same state. If you add more and more neutrons, they have to use states with increasing energy - at some point this makes the nucleus instable against beta decay (a neutron converts to a proton and emits electron+neutrino) or even direct emission of a neutron. Fission of the whole nucleus can get possible as well.