Period 3 Oxides: Structure, Melting Point & Acid-Base Behaviour (AQA A Level Chemistry): Video

Hi, I'm Eleanor with 3 years of experience teaching A-Level Chemistry, and this video is about the structure, melting points, and acid-base behaviour of the Period 3 oxides.

The same structure-and-bonding trend across Period 3 — giant ionic, then giant covalent, then simple molecular — explains both why melting points fall sharply partway across the period and why the oxides shift from basic to acidic, because it's the strength and type of bonding that governs both properties.

Going across Period 3, electronegativity rises, and the bonding in the oxides shifts from ionic — in sodium oxide, magnesium oxide and aluminium oxide — to covalent, in silicon dioxide, phosphorus oxide and the sulfur oxides. That same bonding shift explains the melting point trend: the ionic and giant covalent oxides have very high melting points because their strong bonds extend throughout the whole structure, while the simple molecular oxides melt at much lower temperatures. It also explains what happens when each oxide reacts with water: the metal oxides form alkaline solutions, the oxides in the middle don't react at all, and the non-metal oxides form acidic solutions.

This video covers the melting point trend first, then the structure and bonding behind it, then how the oxides react with water, before finishing with their acid-base reactions.

Sodium oxide, magnesium oxide and aluminium oxide all have high melting points, because their giant ionic lattices need large amounts of energy to overcome the electrostatic attraction between ions. Magnesium oxide has a higher melting point than sodium oxide, because magnesium's 2 plus charge and smaller ionic radius give it a higher charge density, strengthening the attraction to the oxide ion. Aluminium oxide’s malting point is lower than magnesium oxides due to the more covalent character of the bonds in aluminium oxide. Silicon dioxide also has a high melting point, because it's a giant covalent structure with millions of strong covalent bonds. After that, the melting point drops sharply for phosphorus oxide and the sulfur oxides, which exist as small molecules held together by only weak intermolecular forces.

Sodium oxide, magnesium oxide and aluminium oxide are giant ionic structures, formed between a metal and the non-metal oxygen. Silicon dioxide is a giant covalent structure, while phosphorus oxide, sulfur dioxide and sulfur trioxide are simple molecular structures. This shift from ionic to covalent bonding happens because electronegativity increases going from sodium to sulfur across the period.

Sodium oxide and magnesium oxide contain the oxide ion, a strong base, which reacts readily with water to produce hydroxide ions and an alkaline solution. Aluminium oxide and silicon dioxide don't react with water at all: aluminium oxide's ions are held too strongly in its lattice to be separated, and silicon dioxide's covalent bonds are too strong to break. Phosphorus oxide, sulfur dioxide and sulfur trioxide are simple covalent molecules that react with water to form acids, giving acidic solutions.

Sodium oxide and magnesium oxide are basic oxides, reacting with acids like hydrochloric acid to form a salt and water — this is why magnesium oxide is used in indigestion remedies, neutralising excess stomach acid. Aluminium oxide is amphoteric, meaning it can react as both a base with an acid and as an acid with a base. Silicon dioxide, phosphorus oxide and the sulfur oxides are all acidic, reacting with hot, concentrated alkali to form a salt.

The metal oxides at the start of the period form alkaline solutions, the oxides in the middle of the period don't react with water at all, and the non-metal oxides at the end of the period form acidic solutions.

Across Period 3, the bonding in the oxides shifts from ionic to covalent, and that shift explains everything else: melting points stay high while the bonding is giant, whether ionic or covalent, then drop sharply once the oxides become simple molecules. The same shift explains their reaction with water. Metal oxides are alkaline, non-metal oxides are acidic, and the oxides in the middle don't react at all.