Some time ago, I read about a certain isotope that is stable when neutral but decays with electron emission (beta) when being completely ionized, but I can't find which one it was. Which isotope decays when fully ionized?

Does the centre of mass of two planets keep following the same course before and after a collision of two planets in a solar system? Is it possible to use conservation of linear momentum even though there is an external force acting on the planet in some way? If we assume the duration of collisio...