1. The problem statement, all variables and given/known data The 3s state Na has an energy of -5.14eV. Determine the effect nuclear charge. 2. Relevant equations I believe you have to use: En=-[(13.6eV)Zeff^2]/n^2 3. The attempt at a solution So since Na is in the 3s state, I think n=3. So: -5.14eV= -[(13.6eV)Zeff^2)]/9 -46.3eV=(-13.6eV)Zeff^2 Zeff=sqrt(3.40)=1.85 However the answer in the back of the book is giving me 1.14e, could someone show me my mistake?