Electrons fill shells around the nucleus. Each shell holds 2n² electrons — 2, 8, 18, 32.
Walk through
Step 1 of 5
The nucleus
Start with a tiny, heavy nucleus of protons (and usually neutrons). Its positive charge is what holds electrons nearby. The number of protons is the element — 1 = hydrogen, 6 = carbon, 8 = oxygen.
Electrons per shell
n = 1 → 2, n = 2 → 8, n = 3 → 18, n = 4 → 32.
An atom is almost entirely empty space. If the nucleus were a marble at the centre of a football stadium, the electrons would be a faint blur out at the seats.
Watch out
Electrons don't whizz round in neat planetary circles — that's just a handy cartoon. They're spread-out probability clouds (orbitals); the 'shell' is really a set of energy levels, not a track.