Unit Overview
Structure 2: Electron Configuration
IB Chemistry SLΒ· 5 min read π 8-12% of overall exam
1. Unit at a Glance
This unit builds on the simple Bohr atomic model from Unit 1 to introduce the modern quantum understanding of electron location. The learning arc progresses from foundational orbital structure, to the rules that govern electron filling, to observable spectral evidence for the model, ending with how electron configuration explains a key periodic trend. Each sub-topic relies on the content from the previous, so progressing in order is recommended.
This unit is split into 4 connected sub-topics:
Atomic orbitals
Introduces s, p, d, and f orbitals, their shapes, quantum numbers, and energy ordering rules.
β β β± 8 min
Electron configuration rules
Covers Aufbau, Pauli, and Hund's rules for writing full and condensed electron configurations.
β β β β± 10 min
Atomic emission spectra
Explains how electron transitions produce line spectra and how this supports the quantum model.
β β β± 7 min
First ionization energy trends
Connects electron configuration to ionization energy trends across periods and down groups.
β β β β± 9 min
2. Common Pitfalls
Wrong move:
Writing 3d orbitals before 4s in electron configurations
Why:
4s has lower energy than 3d for neutral atoms, so it fills first
Correct move:
Order 4s before 3d, and remove 4s electrons first when forming ions
Wrong move:
Pairing electrons in orbitals before all subshell orbitals are singly filled
Why:
Electrons have lower energy when occupying separate orbitals with parallel spin per Hund's rule
Correct move:
Add one electron to each orbital in a subshell before pairing any electrons
Wrong move:
Assuming higher principal quantum number always means higher orbital energy
Why:
Orbital energy depends on both and the azimuthal quantum number
Correct move:
Use the rule to order orbitals by energy for filling
3. Quick Reference Cheatsheet
Concept / Rule | Key Summary |
|---|---|
Orbital energy ordering rule | rule: lower = lower energy; same , lower = lower energy |
Pauli Exclusion Principle | Maximum 2 electrons per orbital, with opposite spin |
Hund's Rule | Fill degenerate orbitals singly first, with parallel spin, before pairing electrons |
Electron transition energy change | |
First Ionization Energy definition | Minimum energy to remove 1 mole of electrons from 1 mole of gaseous neutral atoms |
Standard neutral atom orbital energy order |
What's Next
Begin this unit with the first sub-topic on atomic orbitals to build the foundational understanding you need for all subsequent content. Once you complete all four sub-topics in this unit, move on to the first sub-topic of the next unit on periodicity.