Properties of Period 3 elements and their oxides (A-level only) — AQA A-Level Study Guide

Overview

This topic explores the physical and chemical properties of the Period 3 elements (Sodium to Argon) and their oxides. It is a fundamental part of A-Level Chemistry because it perfectly illustrates how atomic structure and bonding dictate macroscopic properties. You will learn to explain the dramatic changes in melting points across the period and the shift from basic to acidic behaviour in the oxides.

Examiners frequently test this topic through long-answer (6-mark) questions requiring you to link structure, bonding, and energy. You must also be prepared to write perfectly balanced equations for the reactions of oxides with water, acids, and alkalis. Understanding this topic is crucial as it builds upon your knowledge of atomic structure, periodicity, and acid-base equilibria.

Key Concepts

Concept 1: Reactions of Period 3 Elements with Water

The reactivity of Period 3 elements with water decreases across the period. Sodium reacts vigorously with cold water to form a strongly alkaline solution of sodium hydroxide and hydrogen gas. Magnesium reacts very slowly with cold water to form sparingly soluble magnesium hydroxide, but reacts rapidly with steam to form magnesium oxide and hydrogen.

Why does this happen? Sodium has a lower first ionisation energy than magnesium. It loses its single outer electron more easily than magnesium loses its two outer electrons, making it more reactive.