Electrons Fill by Energy, Not by Number

Fifteen electrons and a nucleus. The electrons do not queue up by shell number — they queue up by height on an energy ladder, and the nucleus sets that height by Coulomb’s law. For the first eighteen elements the two orders agree, which is exactly why the moment they disagree feels like a rule being broken. It isn’t. Watch the ladder fill.

8 STEPS · 6 QUICK CHECKS · PHOSPHORUS Z = 15 · v1