• Figure 1 shows some of the lowest energy levels of the He atom. They are labeled by then...

• Figure 1 shows some of the lowest energy levels of the He atom. They are labeled by then configuration (for example, 1s2p means that the atom has one electron in the 1s level and one in the 2p level). The energy depends somewhat on the orientation of the two electrons’ spins: If the spins are antiparallel, the total spin is zero (quantum number stot = 0); if the spins are parallel, the total spin has stot = 1. For a given configuration, the state with stot = 1 has slightly lower energy. (a) Explain why the 1s2 configuration has only stot = 0. (b) There is a selection rule Δstot = 0, that is, transitions in which stot changes are forbidden. Indicate all allowed transitions on the energy-level diagram. (Don’t forget the selection rule Δl = ±1.) (c) Which excited levels would you expect to be metastable?

FIGURE 1

Some low levels of helium. (These are the energy levels of the whole atom, not the individual electrons.) The numbers 0 and 1 indicate the quantum number stot of the total spiri.