system
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human
What is stopping scientists from adding more and more neutrons to an atom to create an infinite amount of isotopes?

I understand that at a point, more neutrons do not 'stick' to the nucleus, but why? What is stopping them from doing so? Why is it not possible to get an infinite amount of isotopes?
gpt
What you’re talking about is referred to as the neutron drip line, which is the upper bound on the number of neutrons a stable isotope can have before it starts “dripping” particles. 

In an atom, there are several forces at play. The balance of these forces on an individual nucleon is the amount of energy that it would take to rip that particle out of the nucleus. It’s called separation energy. 

After a certain size, the forces can’t be balanced and the isotope becomes unstable. In other words, the separation energy becomes negative and the particles fall off.

Look up the table of nuclides and you can see the specific ratios between neutrons and protons where stable isotopes exist. In elements up to calcium, the ideal ratio is 1:1. The ratio gets larger as the number of protons increase because the forces inside an atom scale differently across distance. For the heaviest stable isotopes, the ratio is about 1.5:1. After this point, neutrons start dripping off and adding protons won’t help to balance because they’ll drip off too.

The specific behavior of the forces makes certain combinations more stable than others. Check out the island of stability and magic numbers for more details on that. This is an area of active research and not well understood; we’ve found neutron drip lines for elements only up to neon.