The photoelectron spectrum of a sodium atom, as in AP Chemistry: one peak for each sublevel, labeled 1s², 2s², 2p⁶ and 3s¹, each as tall as its number of electrons, so the 2p peak is three times as tall as the 1s. Binding energy in MJ/mol falls from left to right on a logarithmic axis, so the tightly held 1s electrons are at the far left and the single valence 3s electron at the far right.
