Electron Shells: Where Electrons Live
Electrons don't orbit anywhere they like — they stack into shells. The outermost shell writes all the chemistry.
Builds on: 0.1 Atoms: The Alphabet of Matter
Floors in the electron hotel
Electrons around a nucleus behave like guests in a strange hotel: they must occupy fixed floors — shells — and each floor has a strict capacity. The first shell (closest to the nucleus) holds at most 2 electrons, the second holds 8, and for the elements you’ll meet first, the third also fills with 8 before anything else happens. Electrons fill from the ground floor up: sodium’s 11 electrons arrange as 2 · 8 · 1.
Only the top floor does chemistry
The electrons in the outermost occupied shell are called valence electrons, and they are the only ones other atoms ever see. Full inner shells are sealed off. That single number — how many electrons sit on the top floor — predicts almost everything: how an element bonds, what it reacts with, and how violently.
The octet rule: everyone wants a full shell
Atoms are at their most stable when their outer shell is full — usually 8 electrons (the octet), or 2 for the tiny first shell. The noble gases (helium, neon, argon…) are born that way, which is why they react with almost nothing: they have nothing to gain. Every other element schemes its way to a full shell by losing, gaining or sharing electrons. Sodium happily dumps its lone outer electron; chlorine hungrily grabs one more. Put them together and you can guess what happens — that story is Unit 1.
Shell capacities come from quantum mechanics (2n² states per shell, and the third shell’s upper rooms only fill after the fourth opens). You don’t need the machinery yet — the 2 · 8 · 8 pattern is exact for the first 20 elements, which is all this course needs until the advanced tier.
In the lab, fill atoms one electron at a time. Watch which shell each electron chooses, then read the verdict: does this element want to lose, gain, or is it already noble?
⚗️ Lab — The Shell Filler
Pick an element, then drag the electron count up from zero and watch the floors fill.
- Fill sodium (11): two floors seal shut, one electron is left exposed on top.
- Fill chlorine (17): the top floor lacks exactly one electron.
- Fill neon (10) and argon (18): full house — that’s why they’re called noble.