Unit Overview

Structure 1: Atomic Structure

IB Chemistry SL· 5 min read 📊 8-12% of overall IB Chemistry SL assessment

1. 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

Introduces the core idea that all matter is made of atoms, and defines elements, compounds, and mixtures.

★⏱ 6 min

Atomic number, mass number and isotopes

Defines subatomic particle properties and distinguishes between different isotopes of the same element.

★★⏱ 7 min

Mass spectrometry

Explains how mass spectrometry works and how to use its output to calculate relative atomic mass.

★★★⏱ 10 min

Basic electron arrangement in atoms

Covers how electrons occupy energy levels and how to write configurations for the first 20 elements.

★★★⏱ 9 min

2. Common Pitfalls

Wrong move:

Confusing mass number and relative atomic mass

Why:

Mass number is an integer for a single isotope, while relative atomic mass is a weighted average across all isotopes.

Correct move:

Remember: mass number = protons + neutrons for one atom; relative atomic mass is the weighted average for an element.

Wrong move:

Claiming isotopes of an element have different chemical properties

Why:

Students often assume different mass equals different reactivity, but chemical behavior depends on electron arrangement.

Correct move:

Isotopes only differ in neutron count and mass; identical electron arrangement means identical chemical properties.

Wrong move:

Swapping atomic number and mass number in nuclide notation

Why:

It is easy to mix up which number goes where in standard notation.

Correct move:

Remember: atomic number (proton count, defines the element) is the lower subscript, mass number is the upper superscript.

3. Quick Reference 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 ( $A_{r}$ )	$A_{r} = \frac{\sum ( isotope mass \times % abundance )}{100}$
Maximum electrons per energy level	$2 n^{2}$ , 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.