# Structure 2: Electron Configuration

> IB Chemistry SL · IB 2025
> Source: https://www.owlsprep.com/study/ib-chemistry-sl-u2-overview/
> Weight: 8-12% of overall exam

This unit explores the quantum mechanical model of the atom, explaining how electrons are arranged in atoms. This arrangement underpins atomic properties, periodic trends, and chemical reactivity.

**Prerequisites:** [Basic atomic structure (protons, neutrons, electrons) and the Bohr model of the atom](https://www.owlsprep.com/study/ib-chemistry-sl-u1-overview/)

## Learning objectives

- Describe the structure, shapes, and energy ordering of atomic orbitals in the quantum mechanical model
- Apply core electron configuration rules to write configurations for neutral atoms and monatomic ions
- Explain how electron energy transitions produce atomic emission spectra and support the quantum model
- Analyze trends in first ionization energy across periods and down groups using electron configuration

## 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](https://www.owlsprep.com/study/ib-chemistry-sl-u2-atomic-orbitals/) — Introduces s, p, d, and f orbitals, their shapes, quantum numbers, and energy ordering rules.
- [Electron configuration rules](https://www.owlsprep.com/study/ib-chemistry-sl-u2-electron-configuration-rules/) — Covers Aufbau, Pauli, and Hund's rules for writing full and condensed electron configurations.
- [Atomic emission spectra](https://www.owlsprep.com/study/ib-chemistry-sl-u2-atomic-emission-spectra/) — Explains how electron transitions produce line spectra and how this supports the quantum model.
- [First ionization energy trends](https://www.owlsprep.com/study/ib-chemistry-sl-u2-first-ionization-energy-trends/) — Connects electron configuration to ionization energy trends across periods and down groups.

## Common pitfalls

- **Wrong:** Writing 3d orbitals before 4s in electron configurations
  - Why it fails: 4s has lower energy than 3d for neutral atoms, so it fills first
  - Correct: Order 4s before 3d, and remove 4s electrons first when forming ions
- **Wrong:** Pairing electrons in orbitals before all subshell orbitals are singly filled
  - Why it fails: Electrons have lower energy when occupying separate orbitals with parallel spin per Hund's rule
  - Correct: Add one electron to each orbital in a subshell before pairing any electrons
- **Wrong:** Assuming higher principal quantum number $n$ always means higher orbital energy
  - Why it fails: Orbital energy depends on both $n$ and the azimuthal quantum number $l$
  - Correct: Use the $(n+l)$ rule to order orbitals by energy for filling

## Cheatsheet

| Concept / Rule | Key Summary |
| --- | --- |
| Orbital energy ordering rule | $(n+l)$ rule: lower $(n+l)$ = lower energy; same $(n+l)$, lower $n$ = 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 | $\Delta E = h\nu = \frac{hc}{\lambda}$ |
| 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 | $1s < 2s < 2p < 3s < 3p < 4s < 3d < 4p$ |

## 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.

- [Atomic orbitals](https://www.owlsprep.com/study/ib-chemistry-sl-u2-atomic-orbitals/)
- [Electron configuration rules](https://www.owlsprep.com/study/ib-chemistry-sl-u2-electron-configuration-rules/)
- [Atomic emission spectra](https://www.owlsprep.com/study/ib-chemistry-sl-u2-atomic-emission-spectra/)

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