Electron configuration
The electron configuration lists which orbitals an atom's electrons fill. Electrons fill the lowest-energy orbitals first, in the order 1s, 2s, 2p, 3s, 3p, 4s, 3d. This is called the Aufbau principle.
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Analogy
Like filling a theatre from the front seats back: lowest energy seats first.
Example
1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁶Short form: [Ar] 4s² 3d⁶
In practice
The configuration tells you how many outer electrons an element has, and so how it reacts.
Practice
Flashcards
- Write the configuration of sulfur (16 electrons).1s² 2s² 2p⁶ 3s² 3p⁴, or [Ne] 3s² 3p⁴.
Interview questions
- Write the electron configuration of Fe and Fe³⁺ and explain which electrons are lost first.
- Fe is [Ar] 4s² 3d⁶
- 4s electrons are lost before 3d
- Fe³⁺ is [Ar] 3d⁵
- Half-filled d subshell is stable
Questions people ask
Why does 4s fill before 3d?
In a neutral atom, the 4s orbital is slightly lower in energy than 3d as filling begins, so it gets electrons first. Once 3d has electrons, the order flips. That is why transition metals lose their 4s electrons first when they form ions.
Why are chromium and copper exceptions?
Chromium is [Ar] 4s¹ 3d⁵ and copper is [Ar] 4s¹ 3d¹⁰. A half-full or full d subshell is slightly more stable, so one 4s electron moves over. The energy gap between 4s and 3d is small, which makes this swap worthwhile.