Chapter 4.1, Problem 4.2P

According to quantum mechanics, the electron cloud for a hydrogenatom in the ground state has a charge density $\rho \left(r\right)=\frac{q}{\pi a^3}{e}^{-2r/a},$ where q is the charge of the electron and a is the Bohr radius. Find the atomicpolarizability of such an atom. [Hint: First calculate the electric field of the electroncloud, $E_e\left(r\right)$ ; then expand the exponential, assuming $r\ll a{.}^1$