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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Periodic TrendsElectron configurationContent level: Core 25 min
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
Lesson
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.
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.
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.
Two exceptions worth knowing
Chromium is [Ar] 4s¹ 3d⁵ and copper is [Ar] 4s¹ 3d¹⁰, not the naive 4s² 3d⁴ and 4s² 3d⁹. The 4s and 3d energies are extremely close in these atoms, and a half-filled or filled d subshell with reduced electron-electron repulsion comes out lower in energy. These two are the only exceptions you are expected to know at this level.
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
In neutral atoms 4s is lower in energy than 3d, so it fills first. This ordering flips once the atom is ionized.
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.
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Common mistakes
Writing 3d before 4s for a neutral atom.
Why it's wrong: In neutral atoms 4s is lower in energy and fills first, so the ground-state configuration lists 4s² before 3d.
Check instead: Follow the table order: the 4s block (period 4, groups 1-2) comes before the 3d block.
Counting filled d electrons as valence electrons for a p-block atom.
Why it's wrong: For arsenic, [Ar] 4s² 3d¹⁰ 4p³, the 3d¹⁰ electrons are buried and do not bond, so arsenic has 5 valence electrons, not 15.
Check instead: Count only the highest-n s and p electrons for main-group elements.
Applying the half-filled stability argument everywhere.
Why it's wrong: Only chromium and copper (and their heavier analogues) are expected exceptions. Manganese is a normal [Ar] 4s² 3d⁵.
Check instead: Assume the standard order unless the element is Cr or Cu.
After the lesson · Question bank
Independent practice
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What you should now know
Electron configurations follow the aufbau order, which the periodic table itself encodes block by block. Noble-gas notation shortens the core, and valence electrons are the ones outside that core.
- 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
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
Access for free at openstax.org License