# Limits of Perturbation theory for hydrogenic atoms

## Homework Statement

Why can't we use perturbation theory to calculate the effect of the spin orbit interaction in hydrogen like uranium?

## The Attempt at a Solution

Is it something to do with the fact that the perturbation must be small compared to the rest of the hamiltonian? But for hydrogen like uranium the ionization energy is still 28000 times that of the spin orbit shift? Or could it be to do with the fact that the bohr radius for uranium approaches the radius of the nucleus?

## The Attempt at a Solution

E_n = <$$\Phi$$_n l H l $$\Phi$$_n>