Originally posted by elas
Appreciate your help and understand your reply but I am looking for the origin of what at present appears to be an assumption.
Given that the electron orbit is determined by magnetic force, why should the addition of a neutral particle alter the orbit but, as mass is being added, why should the volume remain the same?
This can of course be explained as an increase in density but I cannot find any reference to such an increase and even and if I accept a change in density and no change in radii this begs the question 'why does a change in density not alter the electron orbit'?
Given that some isotopes are radioactive (when their associated element is not); clearly there are occassions when the addition of a neutron does alter the magnetic relationship.
If a hydrogen atom was the size of a football field, the proton would be the size of (roughly), a pea. If you put another pea in the field you've doubled the mass but the football field doesn't get any bigger.
You've answered your own second question. The size of an electron's orbit is measured by the electomagnetic force, the mass of the nucleus doesn't have anything to do with it. I don't see the source of your confusion.
What do unstable nuclei have to do with the electromagnetic force and orbitals?