Worked example 1
Write the ground-state electron configuration of sulfur (S, Z = 16) in full and in noble-gas notation.
Try it first: Count 16 electrons across the table, stopping at sulfur.
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Electron configurationCourse level: High School / HonorsCourse level: AP Chemistry
Electron Configurations and Orbital Filling
What you'll be able to do: Write the ground-state electron configuration of any main-group or first-row transition element.
Introduction
Every periodic trend comes from where the electrons sit. This lesson builds the tool you will use for the rest of the unit: a reliable way to write out an electron configuration and read it off the periodic table itself.
These are recommended, not required. You can start this lesson at any time.
Learning objectives
- Write full and noble-gas electron configurations for neutral atoms
- Use the blocks of the periodic table to read filling order directly
- Identify valence electrons and the valence shell from a configuration
- Recognize the chromium and copper exceptions and state why they occur
Subshells hold a fixed number of electrons
Electrons occupy shells (n = 1, 2, 3, ...) that contain subshells labelled s, p, d and f. An s subshell holds 2 electrons, p holds 6, d holds 10 and f holds 14. A configuration is just a tally: which subshells are occupied, and by how many electrons.
Decode 3p⁴ as three separate facts: 3 is the principal shell, p is the subshell, and 4 is its electron population. It does not mean four p orbitals. Each orbital holds two electrons; s, p, d and f contain one, three, five and seven orbitals respectively. A shell's capacity is different from the capacity of one subshell: n = 3 includes 3s, 3p and 3d and can hold 18 electrons, even though period 3 ends after 3p because the next electrons enter 4s.
1s² 2s² 2p⁶ 3s² 3p⁶
The aufbau order, read off the table
Electrons fill from lowest energy upward: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p. You do not have to memorize this list. Walk across the periodic table row by row: groups 1-2 are the s block, groups 13-18 the p block, the middle ten columns the d block. The row number gives n for s and p, and n - 1 for d.
To construct a configuration, first determine the electron count, then fill successive subshells without exceeding their capacities, and stop exactly when the count is exhausted. For phosphorus, Z = 15: 1s² + 2s² + 2p⁶ uses ten electrons, 3s² uses two more, and the remaining three go into 3p³. The p block has six columns because a p subshell holds six electrons. Helium is an important placement exception: it sits with the noble gases but its configuration is 1s², not a p configuration. In longer configurations the f block belongs to n − 2 relative to the period number.
Reading the table beats memorizing the diagonal diagram, because the table is on your reference sheet during the test.
Noble-gas notation
Replace the completed core with the previous noble gas in brackets. Sulfur is 1s² 2s² 2p⁶ 3s² 3p⁴, or [Ne] 3s² 3p⁴. Both are correct, but the short form makes the valence electrons obvious, and the valence electrons are what chemistry actually depends on.
S: [Ne] 3s² 3p⁴
Valence electrons
For a main-group atom, count electrons in its highest occupied principal shell (largest n). Oxygen ([He] 2s² 2p⁴) has six valence electrons in n = 2.
Arsenic ([Ar] 4s² 3d¹⁰ 4p³) has five in n = 4; the filled 3d¹⁰ electrons are not counted as main-group valence electrons even though they appear after [Ar] in the configuration.
For transition metals, d electrons can also take part in bonding, so the simple highest-n shortcut needs care.
For p-block elements after the d block, count only the ns and np electrons as valence.
Chromium and copper: limits of the filling shortcut
The usual filling sequence predicts [Ar] 4s² 3d⁴ for Cr and [Ar] 4s² 3d⁹ for Cu. Their observed ground states are [Ar] 3d⁵ 4s¹ and [Ar] 3d¹⁰ 4s¹. The competing configurations are close in total energy; electron interactions and exchange effects make the observed arrangement lower. A half-filled or filled d subshell is a useful pattern to recognize, but it is not a universal instruction to move an s electron in every atom.
Cr still totals 18 + 5 + 1 = 24 electrons, and Cu totals 18 + 10 + 1 = 29. The configuration changes their distribution, not the electron count or nuclear charge. Writing 3d before 4s instead of after it is an organizational choice, not a different population. Other heavier elements also have exceptions; Cr and Cu are the principal examples used here.
Separate filling order from valence counting
The periodic table gives a reliable route to a neutral atom's configuration, but do not turn that route into a universal rule about which electrons bond or leave first.
Gallium is [Ar] 4s² 3d¹⁰ 4p¹: the filled 3d¹⁰ lies between 4s and 4p in the written configuration, yet its three main-group valence electrons are 4s² 4p¹.
For transition metals, d electrons can participate in bonding and the next lesson explains why 4s electrons are removed first on ionization. Check your configuration by summing every superscript to the atomic number and by identifying the correct block of the table.
Audit an electron configuration
A configuration must pass two independent checks: its superscripts add to the atom's electron count, and the occupied subshells follow the appropriate filling order. A correct total alone cannot catch an electron placed in the wrong subshell.
For a neutral atom, compare the total with its atomic number; for an ion, adjust that count for the charge before checking occupancy.
Count electrons first, then inspect where they sit.
Key ideas
Rule
Subshell capacities
s holds 2, p holds 6, d holds 10, f holds 14 electrons.
Rule
Aufbau principle
Ground-state electrons occupy the lowest-energy orbitals available before higher ones.
Definition
Valence electrons
The electrons outside the noble-gas core that participate in bonding, for main-group atoms the ns and np electrons.
Assumption
4s fills before 3d
The usual neutral-atom filling route reaches 4s before 3d. Actual orbital energies depend on occupation and species; first-row transition-metal cations lose occupied 4s electrons before 3d electrons.
Worked examples
Worked example 2
Write the ground-state electron configuration of iron (Fe, Z = 26).
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Worked example 3
Write the noble-gas configuration of Ga (Z = 31) and count its main-group valence electrons.
Try it first: Move across period 4 from the 4s block through 3d and into 4p.
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Worked example 4
A student writes [Ne] 3s² 3p² for phosphorus. Diagnose the error and repair it.
Try it first: Compare the electron count with phosphorus's atomic number, 15.
0 of 3 steps revealed.
Common mistakes
| Mistake | Why it's wrong | Check instead |
|---|---|---|
| Treating the written order of 3d and 4s as proof of a different configuration. | [Ar] 3d⁶ 4s² and [Ar] 4s² 3d⁶ specify the same electron populations. Written order alone is not an orbital-energy measurement. | Check each population, total electron count and the ground-state configuration of the specified species. |
| Counting filled d electrons as valence electrons for a p-block atom. | For arsenic, [Ar] 4s² 3d¹⁰ 4p³, the 3d¹⁰ electrons are buried and do not bond, so arsenic has 5 valence electrons, not 15. | Count only the highest-n s and p electrons for main-group elements. |
| Applying the half-filled stability argument everywhere. | Other elements also have exceptions, and a half-filled pattern is not a universal algorithm. Cr and Cu are the principal exceptions taught here; Mn normally remains [Ar] 3d⁵ 4s². | Assume the standard order unless the element is Cr or Cu. |
Scroll horizontally if needed to read all columns.
After the lesson · Question bank
Independent practice
Work through new problems on your own. The lesson's quick checks and worked examples stay above if you need to review a step.
35 topic questions across 3 difficulty levels.
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What you should now know
Read the usual filling order from periodic-table blocks, respect subshell capacities, and verify the total electron count. Noble-gas notation shortens a filled core; for main-group atoms, count highest-shell ns and np electrons as valence. Cr and Cu illustrate exceptions that depend on total electronic energy.
- Subshell capacities: s = 2, p = 6, d = 10, f = 14
- The periodic table blocks give the aufbau order without memorization
- Noble-gas notation exposes the valence electrons
- Cr and Cu are the two exceptions expected at this level
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Sources and further reading
- Chemistry 2e, Section 6.4: Electronic Structure of Atoms (Electron Configurations)Flowers, Theopold, Langley, Robinson · OpenStax, Rice University · Chapter 6.4View source
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Source rights and review status - General Chemistry bookshelf (LibreTexts)LibreTexts, University of California, DavisView source
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