Electrons Fill by Energy, Not by Number

You'll learnwhy electrons fill subshells by energy, lowest empty rung first.

Fifteen electrons and a nucleus. They do not queue up by shell number; they queue up by height on an energy ladder, and the nucleus sets that height. Twelve short steps; press Next for each one.

12 STEPS · 6 QUICK CHECKS · PHOSPHORUS Z = 15 · v3

one ladder · lowest empty rung first · height decides

Before you start

What you're looking at

An empty energy ladder with rungs 1s through 3p, beside 15 phosphorus electrons waiting to be placed.

The question

Where does each electron go — and why isn't it simply shell 1, then 2, then 3?

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