# Reactivity 1: Stoichiometry and Periodicity

> IB Chemistry SL · SL
> Source: https://www.owlsprep.com/study/ib-chemistry-sl-u4-overview/
> Weight: 15-20% of overall IB Chemistry SL exam

This unit combines foundational quantitative chemistry (stoichiometry) with core periodic trends and inorganic chemistry, two fundamental pillars that underpin all subsequent topics in IB Chemistry SL.

**Prerequisites:** Fundamental atomic structure and balanced chemical equation writing

## Learning objectives

- Apply the mole concept to calculate amounts of reactants and products in chemical reactions
- Interpret the arrangement of elements in the periodic table based on electronic structure
- Predict periodic trends in key atomic and physical properties across periods and down groups
- Compare the chemical reactivity of common s- and p-block elements based on their periodic position

## Unit at a Glance

This unit splits into two connected core themes: first, quantitative stoichiometry, which gives you the tools to calculate how much of a substance is involved in any chemical reaction, and second, periodicity, which explains how an element's position on the periodic table predicts its physical and chemical behavior.

We start with the mole concept, the foundation of all quantitative chemistry, before moving on to practical calculations for reacting masses, volumes, and concentrations. We then cover the organization of the periodic table, key periodic trends, and end with a survey of common s- and p-block element reactivity.

This unit covers the following core sub-topics:
- [The mole concept](https://www.owlsprep.com/study/ib-chemistry-sl-u4-the-mole-concept/) — Introduces the mole as the SI unit for amount of substance and covers basic molar mass calculations.
- [Reacting masses and volumes](https://www.owlsprep.com/study/ib-chemistry-sl-u4-reacting-masses-and-volumes/) — Teaches how to use mole ratios to calculate masses, gas volumes, and solution concentrations for reacting species.
- [Periodic table classification of elements](https://www.owlsprep.com/study/ib-chemistry-sl-u4-periodic-table-classification-of-elements/) — Explains how the periodic table is organized by electron configuration into periods, groups, and blocks.
- [Periodic trends in atomic properties](https://www.owlsprep.com/study/ib-chemistry-sl-u4-periodic-trends-in-atomic-properties/) — Covers trends in atomic radius, ionization energy, electron affinity, and electronegativity across the periodic table.
- [S- and p-block element chemistry](https://www.owlsprep.com/study/ib-chemistry-sl-u4-s-and-p-block-element/) — Explores the characteristic chemical properties of key groups in the s and p blocks of the periodic table.

## Common pitfalls

- **Wrong:** Confusing molar mass of a substance with the mass of a single particle
  - Why it fails: Molar mass describes mass per mole of particles, not the mass of one individual atom or molecule
  - Correct: Always label molar mass with units of g mol⁻¹ to avoid confusion with mass of a single particle (measured in amu)
- **Wrong:** Assuming all periodic trends hold 100% of the time with no exceptions
  - Why it fails: Subshell electron stability creates small, regularly tested exceptions to key trends like ionization energy
  - Correct: Memorize common exceptions (e.g. ionization energy drop between groups 2 and 13) for exam questions
- **Wrong:** Ignoring state when calculating reaction volumes
  - Why it fails: Molar volume rules only apply to gases, not solids or liquids
  - Correct: Check the state symbol of all species before applying volume stoichiometry calculations

## Cheatsheet

| Concept/Formula | Key Expression/Value |
| --- | --- |
| Mole amount calculation | $n = \frac{m}{M} = \frac{N}{N_A} = cV$ |
| Avogadro's constant ($N_A$) | $6.02 \times 10^{23} \text{ mol}^{-1}$ |
| Molar volume of ideal gas at STP (IB SL) | $22.7 \text{ dm}^3 \text{ mol}^{-1}$ |
| Atomic radius periodic trend | Decreases across a period, increases down a group |
| Ideal gas law | $PV = nRT$ |

## What's next

Begin your study of this unit with the first sub-topic on the mole concept, the foundational quantitative tool for all IB Chemistry. After completing all five sub-topics in this unit, you will be ready to move on to the next unit on bonding and chemical structure, which builds heavily on the periodicity concepts you will learn here.

- [The mole concept (First sub-topic of this unit)](https://www.owlsprep.com/study/ib-chemistry-sl-u4-the-mole-concept/)
- [Reacting masses and volumes](https://www.owlsprep.com/study/ib-chemistry-sl-u4-reacting-masses-and-volumes/)
- [Periodic table classification of elements](https://www.owlsprep.com/study/ib-chemistry-sl-u4-periodic-table-classification-of-elements/)

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