Electron configuration — Science definition

The notation showing how many electrons occupy each sublevel of an atom, such as 1 s² 2 s² 2p⁶.

Where it is taught

Questions using electron configuration

Problem 1

Which is the correct ground-state electron configuration of phosphorus (Z = 15)?
  1. 1 s² 2 s² 2p⁶ 3 s² 3p³
  2. 1 s² 2 s² 2p⁶ 3 s² 3p⁵
  3. 1 s² 2 s² 2p⁶ 3 s² 3d³
  4. 1 s² 2 s² 2p⁸ 3 s² 3p³
Answer and explanation

1 s² 2 s² 2p⁶ 3 s² 3p³

Filling in Aufbau order and stopping at 15 electrons gives 1 s² 2 s² 2p⁶ 3 s² 3p³ (2+2+6+2+3 = 15).

Problem 2

Which is the correct ground-state electron configuration of neon (Z = 10)?
  1. 1 s² 2 s² 2p⁶ 3 s²
  2. 1 s² 2 s² 2p⁶
  3. 1 s² 2 s² 2p⁴
  4. 1 s² 2 s² 2p⁸
Answer and explanation

1 s² 2 s² 2p⁶

Ten electrons fill 1 s² 2 s² 2p⁶ (2 + 2 + 6 = 10), a full second shell.

Problem 3

What is the noble-gas shorthand electron configuration for iron (Z = 26)?
  1. [Ne] 4 s² 3d⁶
  2. [Ar] 4 s² 4d⁶
  3. [Ar] 4 s² 3d⁶
  4. [Ar] 3d⁸
Answer and explanation

[Ar] 4 s² 3d⁶

After [Ar] (18 electrons), the remaining 8 fill 4 s before 3d: 4 s² 3d⁶, giving [Ar] 4 s² 3d⁶.

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