Lesson 4 of 7 · Orbitals & electrons
Electron configuration
An electron configuration shows how an atom’s electrons are arranged among subshells such as 1s, 2s and 2p. Start with the electron count, follow the filling rules, and check for known exceptions. The superscripts must add up to the total number of electrons.
Three rules to apply
Start with the lowest-energy available orbitals; this is the Aufbau approach. Each orbital can hold up to two electrons with opposite spins, following the Pauli exclusion principle. For orbitals of equal energy within a subshell, place one electron in each with parallel spins before pairing them. This is the usual introductory form of Hund’s rule.
For many neutral atoms, the sequence begins 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p. It helps you build a configuration, but orbital energies can change as electrons are added or removed. This matters especially when you write configurations for ions.
| Subshell | Orbitals | Maximum electrons |
|---|---|---|
| s | 1 | 2 |
| p | 3 | 6 |
| d | 5 | 10 |
| f | 7 | 14 |
Reference: OpenStax Chemistry 2e, §6.4: Electron configurations
Check every electron
For a neutral atom, the exponents must sum to its atomic number. For an ion, adjust the total for charge first. Noble-gas notation replaces a filled core with a bracketed symbol: [Ne] represents ten electrons, so [Ne] 3s² represents twelve.
Oxygen’s 2p⁴ arrangement can be written [↑↓] [↑] [↑]. Each box represents one p orbital. The arrows show electron spin states; they do not show electrons moving along a path.
Reference: OpenStax Chemistry 2e, §6.4: Electron configurations
Exceptions and ions
The neutral ground-state configurations of chromium and copper are [Ar] 3d⁵ 4s¹ and [Ar] 3d¹⁰ 4s¹. Treat “half-filled and filled subshells” as a memory aid, not a complete explanation of all exceptions; the total energy includes several competing effects.
When forming common transition-metal cations, remove the outer ns electrons before the (n−1)d electrons. Neutral Fe is [Ar] 3d⁶ 4s²; Fe²⁺ is [Ar] 3d⁶ and Fe³⁺ is [Ar] 3d⁵. Writing 4s before 3d in a neutral-atom filling sequence does not mean 3d must empty first.
Reference: OpenStax Chemistry 2e, §6.4: Electron configurations
Work through the examples
Worked example 1
Write the configuration of oxygen (Z = 8).
- Place two electrons in 1s and two in 2s.
- Place the remaining four in 2p, occupying each p orbital before pairing.
- Check the sum: 2 + 2 + 4 = 8.
Answer: 1s² 2s² 2p⁴, or [He] 2s² 2p⁴.
Worked example 2
Write the configuration of Fe³⁺ (Z = 26).
- Start with Fe: [Ar] 3d⁶ 4s².
- Remove both 4s electrons, then one 3d electron.
- Check: 18 + 5 = 23 electrons, which equals 26 − 3.
Answer: [Ar] 3d⁵.
Avoid these mistakes
- A subshell is not a single orbital unless it is an s subshell.
- Use ground-state data when the simple filling rule fails.
- An orbital picture describes a quantum state, not an electron orbit.
Check your understanding
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Read the complete answer key
1. B — 6. A p subshell has three orbitals, each holding up to two electrons.
2. B — [Ar] 3d⁵ 4s¹. Chromium is a familiar exception to the simple diagonal filling rule.
3. A — 4s. The outer 4s electrons are removed before the 3d electrons for these iron cations.
4. C — 15. The neon core contributes ten, then 2 + 3 gives a total of fifteen.
Common questions
Can I write 4s before 3d?
Yes, configurations may be listed in filling order or grouped by principal shell. What matters is the occupied subshells and electron counts, not just the printed order.
Does the diagonal rule predict every element?
No. Close subshell energies cause exceptions, especially in the d and f blocks. Use verified ground-state configurations rather than treating the mnemonic as exact.
References & model notes
Use these sources to check the definitions and data, or to read more about the topic. The lesson notes where a model or rounded value simplifies the chemistry.