Chemistry · Lesson
Electron configurations and valence electrons
Read a configuration as an electron-count summary for a specified atom or ion. Use filling rules with their scope and check known ground-state exceptions.
Back to the Chemistry mapBy the end, you can
- Count electrons in a configuration.
- Apply Pauli and Hund occupancy rules in a simple orbital diagram.
- Distinguish a useful filling mnemonic from observed ground-state data.
Superscripts count occupancy
An electron configuration names occupied subshells and their populations. Neutral oxygen’s ground-state configuration is 1s² 2s² 2p⁴. The superscripts sum to 2 + 2 + 4 = 8, matching oxygen’s atomic number for a neutral atom. A charge changes the electron-count requirement: a positive monatomic ion has fewer electrons than its Z value.
Noble-gas shorthand replaces an inner configuration with a bracketed symbol. [Ar] represents eighteen electrons in the argon core configuration. It is a notation convenience, not eighteen additional electrons beyond the expanded configuration. Always attach the species and whether the configuration is ground or excited state.
Use boxes and arrows as a ledger
Pauli permits a maximum of two electrons per spatial orbital with opposite spin projections. Hund’s introductory ground-state occupancy rule places electrons in equivalent orbitals singly with parallel spin projections before pairing, within the stated model. For oxygen’s three 2p orbitals and four electrons, an occupancy pattern is [↑↓] [↑] [↑].
The pattern has two unpaired electrons. Any of the equivalent boxes can hold the pair in this schematic presentation. The left-to-right box position is not a measured location. The arrows use the spin-projection convention from the orbital lesson, rather than mechanical rotation or motion through space.
Check the result against the species
A common Aufbau or diagonal filling mnemonic orders subshells to construct many useful introductory ground-state configurations. Real many-electron ground states depend on the energies of the whole configuration and electron interactions. Near-degenerate subshells can make a simple order insufficient.
NIST reports neutral chromium as [Ar] 3d⁵ 4s¹ and neutral copper as [Ar] 3d¹⁰ 4s¹. Their electron counts are 24 and 29. The corresponding singly positive ions have ground-state descriptions [Ar] 3d⁵ and [Ar] 3d¹⁰. Do not explain these observations as literal electron jumps undertaken to satisfy a mnemonic, or claim that 4s always lies below 3d for every occupation and charge state.
| Species | Configuration | Count outside [Ar] | Total electrons |
|---|---|---|---|
| Cr, neutral | [Ar] 3d⁵ 4s¹ | 5 + 1 | 24 |
| Cr⁺ | [Ar] 3d⁵ | 5 | 23 |
| Cu, neutral | [Ar] 3d¹⁰ 4s¹ | 10 + 1 | 29 |
| Cu⁺ | [Ar] 3d¹⁰ | 10 | 28 |
NIST ground-state descriptions for the stated neutral atoms and singly positive ions. [Ar] contributes 18 electrons; compare species-specific data rather than imposing one universal subshell-order story.
Valence language depends on context
For many main-group atoms, the outer occupied principal shell provides a useful introductory valence-electron count. Oxygen then has six outer-shell electrons in 2s² 2p⁴. Those electrons are relevant to chemical bonding, while the inner 1s² pair belongs to the core description.
Transition-metal chemistry can involve both outer s and nearby d electrons, so an outer-shell-only shortcut is not universal. Configurations support explanations of patterns; they do not by themselves supply a complete quantitative prediction of every bond, charge or reaction. Later bonding and inorganic chapters develop those relationships.
Worked examples
Read oxygen’s occupancy
For neutral ground-state oxygen, check the electron count and the simple 2p unpaired-electron count.
- 1s² 2s² 2p⁴ sums to eight electrons.
- Three equivalent 2p orbital boxes first receive parallel single arrows.
- The fourth 2p electron pairs in one box, leaving two singly occupied boxes.
Eight total electrons and two unpaired electrons in this introductory orbital occupancy model.
Try it yourself
How many electrons are described by 1s² 2s² 2p⁴?
Show the answer and reasoning
Eight.
Add the subshell superscripts.
Should you add [Ar]’s 18 electrons to a configuration already fully expanded from 1s?
Show the answer and reasoning
No.
Shorthand replaces the corresponding core terms rather than adding another core.
How many unpaired arrows remain in the stated oxygen 2p⁴ diagram?
Show the answer and reasoning
Two.
One orbital has a pair and two have single arrows under the introductory Hund pattern.
How many electrons are in Cu⁺ described as [Ar] 3d¹⁰?
Show the answer and reasoning
28.
The argon core has 18 and the d subshell has 10; the +1 ion has one fewer electron than neutral copper.
What to carry forward
- Count all occupied-subshell superscripts once.
- Configurations specify the atom or ion and state.
- Orbital boxes and arrows are occupancy conventions.
- Use observed configurations to check a filling mnemonic’s limits.
References
Teaching resources and reference material supporting this lesson’s explanations and examples.
- Chemistry 2e, 6.4 Electronic Structure of AtomsChemistry 2e, 6.4 Electronic Structure of AtomsReference guide
- Chemistry 2e, 6.3 Development of Quantum TheoryChemistry 2e, 6.3 Development of Quantum TheoryReference guide
- Handbook of Basic Atomic Spectroscopic Data: Chromium atomic dataHandbook of Basic Atomic Spectroscopic Data: Chromium atomic dataReference guide
- Handbook of Basic Atomic Spectroscopic Data: Copper atomic dataHandbook of Basic Atomic Spectroscopic Data: Copper atomic dataReference guide
- Handbook of Basic Atomic Spectroscopic Data: Oxygen atomic dataHandbook of Basic Atomic Spectroscopic Data: Oxygen atomic dataReference guide