Electron configuration
Electrons occupy energy levels (1, 2, 3 and so on) around the nucleus. Each level contains sublevels, and each sublevel has a fixed capacity.
| Sublevel | Orbitals | Maximum electrons |
|---|---|---|
| s | 1 | 2 |
| p | 3 | 6 |
| d | 5 | 10 |
Electrons fill the lowest-energy sublevels first, in this order:
1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p
An electron configuration lists each sublevel with its electron count as a superscript. The superscripts add up to the total number of electrons.
Worked example: sodium, 11 electrons
Fill in order until 11 electrons are placed.
- 1s holds 2 (2 so far)
- 2s holds 2 (4 so far)
- 2p holds 6 (10 so far)
- 3s gets the last 1
1s2 2s2 2p6 3s1
Common mistake
Filling 3d before 4s. The 4s sublevel is slightly lower in energy, so it fills first. Potassium is 1s2 2s2 2p6 3s2 3p6 4s1, not 3d1.