1.3.4Additional lessons
Writing Electron Configurations — IB Chemistry HL Revision Notes & Topical Questions
5 min read 4 parts Lesson 11 of 172
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.
Section 1 of 4Rules for Filling
Three rules decide how electrons occupy orbitals.
Aufbau
- Aufbau Fill sublevels from lowest energy upwards: 1s 2s 2p 3s 3p 4s 3d 4p.
Hund
- Hund Orbitals of the same sublevel are occupied singly with parallel spins before any are doubly occupied.
Pauli
- Pauli An orbital holds at most two electrons, which must have opposite spins.
Notation
- Notation Write each sublevel with the number of electrons as a superscript, for example 2p⁴.
Section 2 of 4Writing a Configuration
Follow these steps for any atom, using phosphorus (Z = 15) as an example.
1
Find the number of electrons, which equals the atomic number for a neutral atom: 15.
2
Fill 1s with 2, 2s with 2, 2p with 6, then 3s with 2, and 3p with the 3 left.
3
Write the result: 1s² 2s² 2p⁶ 3s² 3p³.
4
Use the condensed form by replacing the inner noble gas core: [Ne] 3s² 3p³.
5
In orbital boxes, the three 3p electrons occupy three separate orbitals with parallel spins.
Section 3 of 4Exceptions and Ions
Exceptions and Ions
Chromium
- Chromium [Ar] 3d⁵ 4s¹ rather than 3d⁴ 4s², since a half-filled d sublevel gives extra stability.
Copper
- Copper [Ar] 3d¹⁰ 4s¹ rather than 3d⁹ 4s², since a full d sublevel is stable.
Positive ions
- Positive ions Remove electrons from the outermost 4s sublevel first, so Fe²⁺ is [Ar] 3d⁶.
Negative ions
- Negative ions Add electrons to the next available orbital, so O²⁻ is 1s² 2s² 2p⁶.
| Species | Configuration |
|---|---|
| Fe | [Ar] 3d⁶ 4s² |
| Fe³⁺ | [Ar] 3d⁵ |
| Cl⁻ | [Ne] 3s² 3p⁶ |
Section 4 of 4Exam Checklist & Tips
1.3.4 – Key Points for the Exam
- Electron configuration The arrangement of electrons in the sublevels of an atom or ion.
- Hund's rule Electrons occupy degenerate orbitals singly before they pair.
- Condensed configuration A configuration written using a noble gas symbol for the inner electrons.
- Degenerate Describes orbitals with equal energy.
Common Mistakes
- Writing 3d⁴ 4s² for chromium instead of 3d⁵ 4s¹.
- Removing 3d electrons before 4s electrons when forming transition metal ions.
- Pairing electrons in p orbitals before each orbital has one electron.
- Counting the wrong number of electrons for an ion.
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