How many neutrons can an atom have?
No one knows how many neutrons an atom can have.
A nucleus is stable if the number of neutrons does not exceed 1.5 times the number of protons. Nuclei that have too many neutrons are radioactive. But, even if they last for only a fraction of a second, they exist.
There is no good theory to predict the greatest number of neutrons, so it is up to experimentalists to determine the limits.
To date, the nucleus with the highest neutron/proton ratio is
The heaviest known nucleus is