1. The problem statement, all variables and given/known data A hydrogen‑like ion of calcium emits a photon with energy E = 756 eV. What transition was involved? 2. Relevant equations The energy equation: E=Z^2*E_R*(1/n^2-1/n'^2) 3. The attempt at a solution First, Z=20 for a calcium ion, and E_R is the Rydberg energy 13.6 eV. Then I plugged everything in assuming that the electron which emits the photon goes back to the ground state, i.e. n=1. However, I get n' ≈ 1.07, which does not seem sensible to me. Any ideas or suggestions, please? Much appreciated. Note: How do you write down the equations in a neat format on the forums? I'm a newbie so please go easy on me!