# Structure 1: Atomic Structure

> IB Chemistry SL · IB SL Chemistry
> Source: https://www.owlsprep.com/study/ib-chemistry-sl-u1-overview/
> Weight: 8-12% of overall IB Chemistry SL assessment

This foundational unit introduces the core structure of atoms, the building blocks of all matter. All subsequent topics in IB Chemistry rely on the concepts covered in this unit, from bonding to reactivity.

**Prerequisites:** Foundational secondary school general science

## Learning objectives

- Describe the particulate nature of matter and the basic structure of the atom
- Distinguish between atomic number, mass number, and isotopes of an element
- Calculate relative atomic mass from mass spectrometry abundance data
- Write basic electron arrangements for the first 20 elements of the periodic table

## Unit at a Glance

This unit progresses from the most basic idea of matter being made of particles up to the arrangement of electrons inside individual atoms. You will start by learning core definitions, then move to identifying different forms of the same element, explore how we measure atomic masses experimentally, and end with how electrons are organized— a concept critical to understanding all chemical behavior later in the course.

This unit is split into four connected sub-topics:
- [Particulate nature of matter](https://www.owlsprep.com/study/ib-chemistry-sl-u1-particulate-nature-of-matter/) — Introduces the core idea that all matter is made of atoms, and defines elements, compounds, and mixtures.
- [Atomic number, mass number and isotopes](https://www.owlsprep.com/study/ib-chemistry-sl-u1-atomic-number-mass-number-and/) — Defines subatomic particle properties and distinguishes between different isotopes of the same element.
- [Mass spectrometry](https://www.owlsprep.com/study/ib-chemistry-sl-u1-mass-spectrometry/) — Explains how mass spectrometry works and how to use its output to calculate relative atomic mass.
- [Basic electron arrangement in atoms](https://www.owlsprep.com/study/ib-chemistry-sl-u1-basic-electron-arrangement-in-atoms/) — Covers how electrons occupy energy levels and how to write configurations for the first 20 elements.

## Common pitfalls

- **Wrong:** Confusing mass number and relative atomic mass
  - Why it fails: Mass number is an integer for a single isotope, while relative atomic mass is a weighted average across all isotopes.
  - Correct: Remember: mass number = protons + neutrons for one atom; relative atomic mass is the weighted average for an element.
- **Wrong:** Claiming isotopes of an element have different chemical properties
  - Why it fails: Students often assume different mass equals different reactivity, but chemical behavior depends on electron arrangement.
  - Correct: Isotopes only differ in neutron count and mass; identical electron arrangement means identical chemical properties.
- **Wrong:** Swapping atomic number and mass number in nuclide notation
  - Why it fails: It is easy to mix up which number goes where in standard notation.
  - Correct: Remember: atomic number (proton count, defines the element) is the lower subscript, mass number is the upper superscript.

## Cheatsheet

| Concept | Key Fact / Formula |
| --- | --- |
| Isotope definition | Atoms of the same element with same proton number, different neutron number |
| Mass number ($A$) | $A = Z + N$, where $Z$ = atomic number (protons), $N$ = neutrons |
| Relative atomic mass ($A_r$) | $A_r = \frac{\sum (\text{isotope mass} \times \% \text{ abundance})}{100}$ |
| Maximum electrons per energy level | $2n^2$, where $n$ = principal energy level number |
| Neutral atom charge | Number of protons = number of electrons |

## What's next

Start this unit by learning the foundational definitions of the particulate nature of matter, the first sub-topic listed below. These core concepts underpin every other topic in IB Chemistry, so take time to master each section before moving on. Once you have completed all four sub-topics in this unit, you will be ready to move on to the next unit on the periodic table, which builds directly on the atomic structure concepts you learn here.

- [Particulate nature of matter](https://www.owlsprep.com/study/ib-chemistry-sl-u1-particulate-nature-of-matter/)
- [Atomic number, mass number and isotopes](https://www.owlsprep.com/study/ib-chemistry-sl-u1-atomic-number-mass-number-and/)
- [Mass spectrometry](https://www.owlsprep.com/study/ib-chemistry-sl-u1-mass-spectrometry/)

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From [OwlsPrep](https://www.owlsprep.com) — free study guides for A-Level, IB, AP and IGCSE, written against the official syllabus. Canonical page: https://www.owlsprep.com/study/ib-chemistry-sl-u1-overview/