Electron Configuration and Photoelectron Spectroscopy

Filling order and Coulomb's law

Electrons fill from lowest energy: $1s\,2s\,2p\,3s\,3p\,4s\,3d\,4p$. Two exceptions worth knowing: Cr is $[\text{Ar}]4s^13d^5$ and Cu is $[\text{Ar}]4s^13d^{10}$ — a half-full or full $d$ subshell is more stable.

Everything about energy comes back to Coulomb's law: attraction grows with nuclear charge and falls with distance.

Reading a PES spectrum

Each peak is one subshell:

• Peak height $=$ number of electrons in that subshell.
• Peak position $=$ ionisation energy, which tells how tightly held those electrons are.

Core electrons sit far to the high-energy side because they feel almost the full nuclear charge. Valence electrons are shielded and appear at low energy.