Chapter 31 - Atomic Physics - Problems and Conceptual Exercises - Page 1114: 78

Please see the work below.

(a) We know that $K.E=\frac{ke^2}{r}$ and the total energy is given as $E=\frac{-ke^2}{r}$ $\implies K.E=-E$ $\implies K.E=-(-0.85eV)$ $K.E=0.85eV$ (b) We know that $E=K.E+U$ $\implies -0.85=0.85+U$ $\implies U=-2(0.85)eV$ $\implies U=-1.7eV$