Atoms and Shells — the Lone Outer Electron
Electron shells fill in the pattern 2, 8, 18, 32 — and copper ends up with its twenty-ninth electron sitting alone in a shell that could hold thirty-two. It is the furthest out of all of them, and r is squared in Coulomb's law, so a very small nudge sets it free.
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- Press Hydrogen for the simplest case — one electron in one shell
- Press Helium to see a shell filled exactly and tightly bound
- Press Copper to find the twenty-ninth electron alone in the outermost shell
- Compare how far each outer electron sits from its nucleus
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- Shell capacities run 2, 8, 18, 32, matching the 2n² rule
- Copper's outer shell holds one electron where it could hold thirty-two
- Helium's filled shell holds its electrons tightly and gives up none
- Doubling the distance from the nucleus quarters the Coulomb attraction
- The single loosely held outer electron per atom is what makes copper a good conductor
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Shell capacity follows 2n², giving 2, 8, 18 and 32 for the first four. Copper's twenty-nine electrons fill the first three shells completely and leave a single electron alone in the fourth. The Coulomb attraction holding an electron to the nucleus is F = ke²/r², so it falls off as the square of the distance; the lone outer electron is both the furthest from the nucleus and screened by the twenty-eight inside it. Its binding energy is correspondingly tiny, which is why copper releases roughly one free electron per atom at room temperature.