01Reading a PES graph
PES knocks electrons out with high-energy photons and measures how much energy each needed (binding energy). Each peak is a subshell; peak height is the number of electrons in it.
Peaks far left (high binding energy) are core electrons close to the nucleus. Peaks to the right are valence electrons.
02Notebook box
Peak position → subshell energy. Peak height → electron count (ratio 2 : 2 : 6 …).
Worked example: Peaks with relative heights 2, 2, 6, 2, 1 → 1s² 2s² 2p⁶ 3s² 3p¹ → aluminum.