Every element’s chemistry comes down to where its electrons sit. This guide walks through shells, subshells and the rules that decide the order they fill in, so you can write the configuration of any element in the first four periods.
Shells and subshells
Electrons occupy shells numbered n = 1, 2, 3 and so on. Each shell splits into subshells labelled s, p, d and f. A subshell has a fixed number of orbitals, and each orbital holds at most two electrons with opposite spins.
| Subshell | Orbitals | Max electrons |
|---|---|---|
| s | 1 | 2 |
| p | 3 | 6 |
| d | 5 | 10 |
| f | 7 | 14 |
A configuration lists the occupied subshells with the number of electrons as a superscript. Hydrogen is 1s¹, helium is 1s².
Q: How many electrons fit in a 3d subshell?
A: Ten. A d subshell has five orbitals with two electrons each.
The filling order
Electrons fill the lowest-energy subshell available first. This is called the Aufbau principle. The order is not strictly by shell number: 4s fills before 3d, and 5s before 4d.
The order to remember
- 1s, 2s, 2p
- 3s, 3p
- 4s, 3d, 4p
- 5s, 4d, 5p
You don’t need to memorise it: the periodic table is the map. Groups 1–2 are the s-block, groups 13–18 the p-block, and the transition metals in between are the d-block. Reading the table left to right, top to bottom, gives the filling order.
Q: Which fills first, 4s or 3d?
A: 4s. That is why potassium and calcium put their outer electrons in 4s before the transition metals start on 3d.
Hund’s rule
Within a subshell, electrons spread out across the orbitals with parallel spins before any of them pair up. Compare carbon, nitrogen and oxygen:
[orbitals] C | Carbon | 1s2 2s2 2p2 | two unpaired electrons in separate 2p orbitals
[orbitals] N | Nitrogen | 1s2 2s2 2p3 | three unpaired electrons, a half-filled 2p
[orbitals] O | Oxygen | 1s2 2s2 2p4 | the fourth 2p electron has to pair up
Q: How many unpaired electrons does nitrogen have?
A: Three, one in each 2p orbital.
Noble gas shorthand
Long configurations repeat the core of the previous noble gas. Write that noble gas in square brackets and then only what comes after it. Sodium is [Ne] 3s¹ rather than 1s² 2s² 2p⁶ 3s¹.
[orbitals] Na | Sodium | 1s2 2s2 2p6 3s1 | a single valence electron in 3s
Q: What is calcium’s configuration in shorthand?
A: [Ar] 4s².
Exceptions
Chromium and copper break the pattern. A half-filled or completely filled d subshell is especially stable, so one electron moves from 4s into 3d.
- Chromium: expected [Ar] 4s² 3d⁴, actual [Ar] 4s¹ 3d⁵
- Copper: expected [Ar] 4s² 3d⁹, actual [Ar] 4s¹ 3d¹⁰
[orbitals] Cr | Chromium | 1s2 2s2 2p6 3s2 3p6 4s1 3d5 | six unpaired electrons
Q: Why does copper end in 4s¹ 3d¹⁰ rather than 4s² 3d⁹?
A: A completely filled 3d subshell is lower in energy, which outweighs the cost of moving one electron out of 4s.