Subjects and lessons to explore.

Law

General Chemistry I · Atomic Structure

Electron Configurations

7 min read
8 key termsReport an error
Want it in plain words first? Jump to Eli explains — the same idea, no jargon.
On this page 7 sections
  1. In 30 seconds
  2. Why this matters
  3. The college version
  4. Eli explains
  5. Worked example
  6. Key takeaway
  7. Study tools

In 30 seconds

An atom's lists how its electrons occupy orbitals, built by three rules: the (fill lowest-energy orbitals first), the (max two electrons per orbital, opposite spins), and (fill degenerate orbitals singly before pairing). Configurations can be written in full (1s²2s²…) or abbreviated with the nearest noble gas core ([Ne]3s²…). Chromium and copper are famous exceptions because half-filled and filled d subshells are unusually stable.

Why this matters

Unpaired electrons determine whether a substance is paramagnetic (attracted to a magnetic field) or diamagnetic (weakly repelled). Oxygen's two unpaired electrons make O₂ paramagnetic — a property exploited by the magnetic analyzers used in ventilators and anesthesia machines to measure inspired oxygen concentration. The electron configurations of transition metals also underlie essential physiology: the partially filled d orbitals of iron give hemoglobin its oxygen-binding ability, and the configurations of many metal ions dictate the colors and catalytic activity of metalloenzymes. Predicting unpaired electrons from an is a standard exercise in both general chemistry and medical physics.

The college version

1. The Three Filling Rules

  • Aufbau principle ("building up"): electrons occupy the lowest-energy orbitals available first. The filling order follows the diagonal diagram: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p. Note that 4s fills before 3d (4s is lower in energy for neutral atoms).
  • Pauli exclusion principle: no two electrons in an atom can have the same four quantum numbers. Since an orbital fixes n, l, and m_l, the two electrons in one orbital must have opposite spins (m_s = +½ and −½). Each orbital therefore holds a maximum of two electrons.
  • Hund's rule: when electrons occupy degenerate (equal-energy) orbitals, such as the three p orbitals, they fill them singly with parallel spins before any pairing occurs. This minimizes electron–electron repulsion.

2. Full and Noble-Gas-Core Configurations

A full electron configuration lists every occupied subshell, for example sodium: 1s² 2s² 2p⁶ 3s¹. An abbreviated (noble-gas-core) configuration replaces the inner, filled shells with the symbol of the preceding noble gas in brackets. Sodium's core is neon, so sodium is written [Ne]3s¹. The bracketed noble gas represents all electrons up to that noble gas; only the (outermost shell) are written explicitly.

3. Orbital Diagrams and the Cr/Cu Anomalies

An orbital diagram represents each orbital as a box (or line) and each electron as an arrow (↑ or ↓). For example, nitrogen (1s² 2s² 2p³) shows two paired arrows in 1s and 2s, and three single ↑ arrows spread across the three 2p boxes.

Two transition-metal anomalies arise from the special stability of half-filled (d⁵) and filled (d¹⁰) subshells:

  • Chromium (Cr, Z = 24): predicted [Ar]4s² 3d⁴, but actually [Ar]4s¹ 3d⁵.
  • Copper (Cu, Z = 29): predicted [Ar]4s² 3d⁹, but actually [Ar]4s¹ 3d¹⁰.

In both cases one 4s electron is promoted into the 3d subshell to achieve the more stable half-filled or filled d arrangement.

How it works

  1. Determine the number of electrons (the atomic number Z for a neutral atom).
  2. Fill orbitals in Aufbau order (diagonal rule): 1s → 2s → 2p → 3s → 3p → 4s → 3d → 4p → …
  3. Place no more than two electrons per orbital, with opposite spins (Pauli).
  4. Within degenerate sets (p, d, f), fill singly first with parallel spins (Hund).
  5. Optionally abbreviate by replacing the filled core with the preceding noble gas symbol, and note any exceptions (Cr, Cu, and related ions).

Common confusions

Do not confuseWithDifference
Hund's rulePauli exclusion principleHund's governs filling among degenerate orbitals; Pauli limits each orbital to two opposite-spin electrons.
4s before 3d3d before 4sFor neutral atoms 4s fills before 3d (though 3d is conventionally written first in noble-gas notation after filling).
Valence electronsTotal electronsValence electrons are only those in the outermost shell.
Orbital diagramElectron configurationThe diagram shows spins with arrows; the configuration lists subshell occupancies.

Memory aids

"Aufbau Adds up, Pauli Pairs (opposite spins), Hund Half-fills first." For the two exceptions: "Chromium and Copper are Crazy — they steal one electron from 4s to half-fill (Cr, d⁵) or fill (Cu, d¹⁰) the d subshell."

Quick review

Topic Recap

Electron configurations are built by three rules: Aufbau (lowest energy first), Pauli (two electrons per orbital, opposite spins), and Hund (single-fill before pairing). They are written in full (1s²2s²…) or noble-gas-core form ([Ne]3s²…) and shown visually with orbital diagrams. Chromium and copper are the classic exceptions, preferring half-filled (3d⁵) and filled (3d¹⁰) d subshells. These configurations set the stage for effective nuclear charge and periodic trends, the subject of the next topic.

Knowledge Check

  1. State Hund's rule and give a one-word reason for it.
  2. Write the full electron configuration of magnesium (Z = 12).
  3. Write the noble-gas configuration of chlorine (Z = 17).
  4. What is the actual electron configuration of copper, and why does it differ from the predicted one?
  5. How many unpaired electrons does nitrogen (Z = 7) have?

Answers and Rationales

  1. Degenerate orbitals are filled singly with parallel spins before pairing, to minimize electron–electron repulsion.
  2. 1s² 2s² 2p⁶ 3s² (12 electrons).
  3. [Ne]3s² 3p⁵ (neon core + 7 valence electrons).
  4. [Ar]4s¹ 3d¹⁰ — a filled d subshell is more stable than the predicted [Ar]4s² 3d⁹.
  5. Three — nitrogen is 1s² 2s² 2p³, and Hund's rule puts one electron in each of the three 2p orbitals.
Eli, the EliExplains learning guide

Eli explains

The same idea, in plain words

Explain it like I’m 10

Filling orbitals is like seating people on a bus. People (electrons) always take the front seats first — the lowest-energy orbitals — which is the Aufbau principle. Two people can share a bench seat, but only if one faces forward and one faces backward — that's the Pauli exclusion principle (opposite spins). And when there are several empty seats side by side, people spread out and take one seat each before doubling up with a stranger — that's Hund's rule.

So for the "bus" of the carbon atom, its six electrons fill the two front seats (1s), then the next two (2s), and the last two spread out into two different 2p seats rather than sitting together.

This comparison stops being exact because the "bus" of an atom does not fill strictly front-to-back for every element. For a few elements, a seat slightly farther back is slightly more comfortable than the front row, so electrons rearrange themselves — the anomalies of chromium and copper. Real electron behavior also involves repulsion between electrons, which a simple seating chart ignores.

Simple Example

Oxygen has 8 electrons: 1s² 2s² 2p⁴. In the 2p subshell there are three empty "seats" (orbitals). By Hund's rule, the four 2p electrons fill two orbitals singly and pair up in only one of them. This is why oxygen's orbital diagram shows two unpaired electrons — the key to its magnetic behavior.

Worked example

Electron configurations use no energy equations, but they follow the counting rules above. Three short calculations illustrate the logic.

Example 1 — counting electrons. Phosphorus has Z = 15 electrons. Write its full and noble-gas configurations and its valence-electron count.

  1. Fill in order: 1s² (2), 2s² (4), 2p⁶ (10), 3s² (12), 3p³ (15). Full: 1s² 2s² 2p⁶ 3s² 3p³.
  2. The preceding noble gas is neon (10 electrons), so the core is [Ne]: [Ne]3s² 3p³.
  3. Valence electrons = 3s² 3p³ = 5.

(Common error: writing 3p⁵ or miscounting, which changes the valence-electron count.)

Example 2 — electrons in a subshell. How many electrons are in the 3d subshell of iron (Fe, Z = 26)?

  1. Fill to iron: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁶ (total 2+2+6+2+6+2+6 = 26). Note 4s fills before 3d.
  2. The 3d subshell holds 6 electrons: Fe is [Ar]4s² 3d⁶.

(Common error: filling 3d before 4s, which still gives 6 d electrons here but misstates the 4s occupation — for neutral atoms 4s fills first.)

Example 3 — unpaired electrons by Hund's rule. How many unpaired electrons does sulfur (S, Z = 16) have?

Sulfur: 1s² 2s² 2p⁶ 3s² 3p⁴. The 3p⁴ electrons fill three orbitals as ↑↓, ↑, ↑ (two paired, two unpaired). Sulfur has 2 unpaired electrons.

Key takeaways

  • High yield: Aufbau = lowest energy first; Pauli = two per orbital, opposite spins; Hund = single-fill before pairing.
  • High yield: Filling order: 4s fills before 3d for neutral atoms.
  • High yield: Cr is [Ar]4s¹ 3d⁵; Cu is [Ar]4s¹ 3d¹⁰.
  • Subshell capacities: s = 2, p = 6, d = 10, f = 14.
  • Noble-gas-core notation puts the preceding noble gas in brackets and writes only valence electrons after it.
  • Unpaired electrons (from orbital diagrams) explain paramagnetism; fully paired configurations are diamagnetic.
  • Valence electrons determine an element's chemistry; group number often equals valence-electron count for main-group elements.

Keep learning

Ready to build on this? Continue to the next lesson.

Practice General Chemistry I

This lesson has no separate scored set. Practice draws from the subject’s question bank.

Study tools & related lessonsYou’ll learn to · Key vocabulary · Related

You’ll learn to

  • State the Aufbau principle, the Pauli exclusion principle, and Hund's rule.
  • Write full and noble-gas-core electron configurations for atoms and ions.
  • Draw orbital diagrams showing electron spin.
  • Explain the electron-configuration anomalies of chromium and copper.

Key vocabulary

Electron configuration
The distribution of electrons among orbitals
Aufbau principle
Fill lowest-energy orbitals first
Pauli exclusion principle
Max two electrons per orbital with opposite spins
Hund's rule
Fill degenerate orbitals singly before pairing
Orbital diagram
Boxes (orbitals) with arrows (electrons)
Noble-gas-core configuration
Abbreviated form using a bracketed noble gas
Valence electrons
Electrons in the outermost shell
Anomalous configuration
Exception to the standard filling order (Cr, Cu)

Educational content only. It is not medical, legal or professional advice. Found an error? Tell us.