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Sodium and magnesium oxides are ionic lattices; aluminium oxide has a giant structure with significant covalent character. Silicon dioxide is giant covalent. Phosphorus and sulfur oxides are molecular. These structural changes explain large differences in melting point. In a giant lattice, many strong attractions must be overcome; between molecular oxides the relevant attractions are intermolecular. Do not assume an oxide has the same structure as its parent element or that every covalent oxide is a simple molecule.
Video by Eliot Rintoul. The video’s own board label is retained; follow the specification reference and scope notes on this lesson for assessed content.
Viewing focus: Explain structure and melting points of Period 3 oxides
After watching, attempt these questions before opening the answers. You can also use the written lesson below.
After watching 1: Which oxide is amphoteric?
Aluminium oxide. It reacts with both acids and strong alkalis.
After watching 2: Which structure does SiO2 have?
Giant covalent. An extended covalent network accounts for its high melting point.
What you'll be able to do
Sodium and magnesium oxides are ionic lattices; aluminium oxide has a giant structure with significant covalent character. Silicon dioxide is giant covalent. Phosphorus and sulfur oxides are molecular. These structural changes explain large differences in melting point. In a giant lattice, many strong attractions must be overcome; between molecular oxides the relevant attractions are intermolecular. Do not assume an oxide has the same structure as its parent element or that every covalent oxide is a simple molecule.
Sodium oxide and magnesium oxide are basic and react with acids to form salts and water. Sodium oxide reacts readily with water, while magnesium oxide is only sparingly soluble. Aluminium oxide is amphoteric: it reacts with both acids and strong alkalis. Silicon dioxide is an acidic oxide despite being insoluble in water; it reacts with hot concentrated alkali. Acidic molecular oxides such as sulfur trioxide react with water to form acids. Solubility and acid–base classification are related but are not the same property.
To explain amphoterism, provide one acid reaction and one alkali reaction with valid products. For sodium oxide with water, oxide accepts protons to form hydroxide ions. Sulfur dioxide and sulfur trioxide correspond to different sulfur oxidation states and form different acids with water. An unfamiliar oxide question can be approached by identifying its structure, oxidation state and likely acid–base role, then balancing atoms and charge. Experimental evidence, including solubility and pH, should support rather than replace the chemical equation.
Attempt these problems on paper before opening the worked reasoning. Explain your steps as well as giving a final answer.
Common mistakes to avoid
Key takeaways
Question 1: Explain why insoluble silicon dioxide can still be called acidic.
SiO2 + 2NaOH → Na2SiO3 + H2O.
Check the assessed scope and practical or mathematical requirements.
An external explanation or interactive activity to use alongside the lesson. It may include extension material.
Close the notes and answer from memory. Revisit any explanation you cannot yet reconstruct.
Test yourself
Answer a few questions, then review anything you missed.
Start with the quiz above to check the key ideas. You can also choose another topic from the practice library.