We know that the proton/neutron ration must be in a certain range for the atom to be stable. If you add too many protons, they repel each other & you get instability. Now say you add too many neutrons. Why should that make the atom unstable? Neutrons add to the strong force & neutorns separate the protons from each other. So it seems that the more heutons you add, the more stable the atom should get. But it doesn't! Too many neutrons create instability. WHY?
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Free neutrons (not bound inside a nucleus) decay with a half life of a few minutes. (10mins? 15mins?).
Inside a nucleus, neutrons are stable because their decay would result in the production of protons; this is inhibited by the presence of the protons already in the nucleus.. Here's a quote from the link:
"When bound inside of a nucleus, the instability of a single neutron to beta decay is balanced against the instability that would be acquired by the nucleus as a whole if an additional proton were to participate in repulsive interactions with the other protons that are already present in the nucleus. As such, although free neutrons are unstable, bound neutrons are not necessarily so. The same reasoning explains why protons, which are stable in empty space, may transform into neutrons when bound inside of a nucleus."
If there are excess neutrons in a nucleus, then we have a situation midway between free (unstable) neutrons and bound (stable) ones - so the excess neutrons are liable to gradually decay as if they were free.
Inside a nucleus, neutrons are stable because their decay would result in the production of protons; this is inhibited by the presence of the protons already in the nucleus.. Here's a quote from the link:
"When bound inside of a nucleus, the instability of a single neutron to beta decay is balanced against the instability that would be acquired by the nucleus as a whole if an additional proton were to participate in repulsive interactions with the other protons that are already present in the nucleus. As such, although free neutrons are unstable, bound neutrons are not necessarily so. The same reasoning explains why protons, which are stable in empty space, may transform into neutrons when bound inside of a nucleus."
If there are excess neutrons in a nucleus, then we have a situation midway between free (unstable) neutrons and bound (stable) ones - so the excess neutrons are liable to gradually decay as if they were free.