Originally posted by AThousandYoung
That's what I was looking for.
Why is 138 not classically stable?
Apparently the fine structure constant is approximately 137.036, which figures into the following expression for classical (non-relativistic) electron speed:
v = Z*c/137.036
where Z is the atomic number of the element. When Z = 137, the speed is still slightly less than the speed of light, but if Z = 138 or more, the speed becomes superluminal. When you take relativistic effects into consideration, apparently 138 becomes OK too.