What Is Electron Configuration? A Guide to Writing Electron Configurations
Electron configuration shows how an atom's electrons are distributed across shells and subshells. For example, sodium's (atomic number 11) electron configuration is written as 1s² 2s² 2p⁶ 3s¹. On the interactive periodic table you can see the electron configuration and last shell of every element.
Shells and subshells
Electrons occupy shells numbered n = 1, 2, 3, … Each shell has a number of subshells, labelled s, p, d and f. Their capacities are:
- An s subshell: 2 electrons
- A p subshell: 6 electrons
- A d subshell: 10 electrons
- An f subshell: 14 electrons
The maximum number of electrons in a shell is 2n²; that is, shells 1 through 4 hold at most 2, 8, 18 and 32 electrons respectively.
Three rules for writing electron configuration
- The Aufbau principle: electrons fill the lowest-energy subshells first.
- The Pauli exclusion principle: each orbital holds at most two electrons, with opposite spins.
- Hund's rule: within equal-energy orbitals, electrons first occupy them singly, with the same spin, before pairing up.
Subshell filling order
1s → 2s → 2p → 3s → 3p → 4s → 3d → 4p → 5s → 4d → 5p → 6s → 4f → 5d → 6p → 7s → 5f → 6d → 7p
Note that the 4s subshell fills before 3d, but when writing the configuration the subshells are usually ordered by shell number instead (…3d then 4s).
Examples
Three steps to write an element's configuration: 1) find its atomic number. 2) place the electrons into subshells in filling order. 3) check that the electron total matches the atomic number.
| Atomic no. | Element | Full configuration | Short configuration | Note |
|---|---|---|---|---|
| 11 | Sodium (Na) | 1s² 2s² 2p⁶ 3s¹ | [Ne] 3s¹ | One electron in the last shell |
| 17 | Chlorine (Cl) | 1s² 2s² 2p⁶ 3s² 3p⁵ | [Ne] 3s² 3p⁵ | Seven electrons in the last shell |
| 26 | Iron (Fe) | 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁶ 4s² | [Ar] 3d⁶ 4s² | The 3d subshell is written before 4s |
| 24 | Chromium (Cr) | 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁵ 4s¹ | [Ar] 3d⁵ 4s¹ | An exception |
| 29 | Copper (Cu) | 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s¹ | [Ar] 3d¹⁰ 4s¹ | An exception |
Short configuration and the last shell
To shorten a configuration, the part shared with the nearest preceding noble gas is written as its symbol in brackets; for example, sodium's is [Ne] 3s¹. The last (or valence) shell is an atom's outermost electron shell, and it determines the element's chemical properties and group; that is why elements in the same group share a similar last shell.
Exceptions to electron configuration
Some elements' configurations do not match the simple filling order. Textbooks usually explain this by the extra stability of a half-filled or fully filled d subshell; for example, chromium's configuration is [Ar] 3d⁵ 4s¹ instead of [Ar] 3d⁴ 4s², and copper's is [Ar] 3d¹⁰ 4s¹. See these elements: Chromium (Cr) and Copper (Cu).
Frequently asked questions
What is electron configuration?
Electron configuration shows which shells and subshells an atom's electrons occupy; for example, sodium's electron configuration is 1s² 2s² 2p⁶ 3s¹.
What is the filling order of the subshells?
The subshells fill by increasing energy: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p.
How many electrons does each subshell hold?
An s subshell holds 2 electrons, p holds 6, d holds 10 and f holds 14.
What is the short (noble-gas) configuration?
In the short form, the part shared with the nearest preceding noble gas is written as its symbol in brackets; for example, sodium's short configuration is [Ne] 3s¹.