(a) Write an expression for the energy of a single electron in a hydrogen-like orbit. Your expression should take into account a nuclear charge of Z and quantum number n.(b) Express your answer in Hartrees if the electron is in a hydrogen 1s orbital. (c) Calculate the energy in Hartrees of an electron in a helium 1s and a helium 2s orbital. (d) Neglecting the r12 term in the Hamiltonian, calculate in Hartrees the energy of helium excited state having one electron in a 1s orbital and the other electron in a 2s orbital.