Writing electron configurations
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Flip to reveal answersState Aufbau's principle.
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Question
State Aufbau's principle.
Answer
Electrons fill the **lowest-energy** sub-shell available first (build up: 1s, 2s, 2p, 3s, …).
Question
State the Pauli exclusion principle.
Answer
Each orbital holds **at most 2 electrons**, and they must have **opposite spins**.
Question
State Hund's rule.
Answer
Within a sub-shell, electrons occupy orbitals **singly with parallel spins** before any pairing occurs.
Question
What is the sub-shell filling order across the first four rows?
Answer
1s, 2s, 2p, 3s, 3p, **4s, 3d**, 4p — note **4s fills before 3d**.
Question
Max electrons in s, p and d sub-shells?
Answer
**s = 2**, **p = 6**, **d = 10** (each orbital holds 2).
Question
Full electron configuration of a sulfur atom (Z = 16)?
Answer
1s² 2s² 2p⁶ 3s² 3p⁴.
Question
What is a condensed (core) configuration?
Answer
Replace the inner electrons with the **previous noble gas** in [ ], then list the outer electrons — e.g. Ca = [Ar] 4s².
Question
How do you write a positive-ion configuration?
Answer
Start from the atom and **remove electrons from the highest main shell (largest n) first** — for transition metals, **4s before 3d**.
Question
Configuration of Fe²⁺ (Fe is [Ar] 3d⁶ 4s²)?
Answer
**[Ar] 3d⁶** — the two **4s** electrons are removed first, not the 3d.
Question
How do you write a negative-ion configuration?
Answer
**Add** the gained electrons to the next available sub-shell — e.g. O²⁻ = 1s² 2s² 2p⁶.
Question
Why is chromium [Ar] 3d⁵ 4s¹?
Answer
A **half-full** 3d⁵ sub-shell is extra stable, so one 4s electron promotes to 3d.
Question
Why is copper [Ar] 3d¹⁰ 4s¹?
Answer
A **full** 3d¹⁰ sub-shell is extra stable, so one 4s electron promotes to 3d.
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Electron configurations
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