Why are bonded ions more stable than separate ions?
What's meant by stability is whether the compound &quot;wants&quot; to be in current state. If it wants to change the state (separated ions want to bond) then the system is less stable.
In your example you can simply imagine 2 spherical magnets placed close to each other - they would rush towards one another. They had potential energy that turned into kinetic energy. And once they met - they don't have potential energy anymore, thus they are at lower energy state (more stable) and don't want to separate. Then you'd need to apply force to separate them.