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1.3.4Additional lessons

Writing Electron Configurations — IB Chemistry SL Revision Notes & Topical Questions

5 min read 5 parts Lesson 10 of 98

📚 IB Chemistry SL Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Part 1 of 5Three Rules

The same three principles decide how electrons are arranged.

Aufbau principle
  • Aufbau principle Electrons fill the lowest energy sublevel available first, then move to higher ones.
Pauli exclusion
  • Pauli exclusion An orbital can hold at most two electrons, and these must have opposite spins.
Hund's rule
  • Hund's rule Orbitals of the same sublevel are occupied singly with parallel spins before any orbital receives a second electron.
Condensed form
  • Condensed form The inner electrons can be replaced by the symbol of the previous noble gas in square brackets.

Part 2 of 5Writing a Configuration

Follow the order of filling and count the electrons.

1
Find the number of electrons from the atomic number of the neutral atom.
2
Fill sublevels in order: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, making sure that the total adds up to the atomic number.
3
Write the superscript for the number of electrons in each sublevel, such as 2p⁴ for four electrons.
4
Check the sum of the superscripts equals the atomic number of the atom, adjusted for charge in an ion.

Part 3 of 5Exceptions and Ions

Exceptions and Ions
Chromium
  • Chromium Has the configuration [Ar] 4s¹ 3d⁵, not 4s² 3d⁴, because a half-filled d sublevel is more stable.
Copper
  • Copper Has the configuration [Ar] 4s¹ 3d¹⁰, because a completely filled d sublevel is more stable.
Positive ions of d-block
  • Positive ions of d-block Lose electrons from the 4s sublevel first, then from 3d, even though 4s fills first.
Negative ions
  • Negative ions Gain electrons into the next available orbital, so the chloride ion has the argon configuration.
SpeciesConfiguration
O (Z = 8)1s² 2s² 2p⁴
Cl (Z = 17)[Ne] 3s² 3p⁵
Fe (Z = 26)[Ar] 4s² 3d⁶
Fe³⁺[Ar] 3d⁵

Part 4 of 5Worked Example

Question Write the full electron configuration of the Fe³⁺ ion (Z = 26).

  1. Neutral Fe has 26 electrons: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁶.
  2. Fe³⁺ has lost three electrons, giving 23 in total.
  3. The two 4s electrons are lost first, then one 3d electron: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁵.

Answer 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁵, or [Ar] 3d⁵

Part 5 of 5Exam Checklist & Tips

1.3.4 – Key Points for the Exam

  • Aufbau principle Electrons occupy the lowest energy orbitals available first.
  • Hund's rule Electrons occupy degenerate orbitals singly before pairing.
  • Pauli exclusion principle An orbital holds at most two electrons, and these must have opposite spins.
  • Condensed configuration A configuration that uses the previous noble gas symbol in brackets for the inner electrons.

Common Mistakes

  • Removing 3d electrons before 4s when forming transition metal cations.
  • Writing chromium as [Ar] 4s² 3d⁴ and copper as [Ar] 4s² 3d⁹.
  • Pairing electrons in a p sublevel before each orbital has one electron.
  • Writing a configuration whose electrons do not add up to the atomic number, less any ion charge.

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