Which of the following oxides has a giant covalent structure?

Step 1: Analyze the question We need the oxide whose atoms are all joined by covalent bonds in a continuous, three-dimensional network (a giant covalent or macromolecular structure).

Step 2: Concept: structure of oxides across Period 3

• Na₂O, MgO (metals on the left): giant ionic lattices, basic oxides.
• Al₂O₃: ionic with some covalent character, amphoteric oxide, still treated as a giant ionic lattice at WASSCE level.
• SiO₂: giant covalent (macromolecular) structure.
• P₄O₁₀, SO₂, SO₃ (non-metals on the right): simple molecular covalent structures, acidic oxides.

Step 3: Check each option

• A. Al₂O₃: giant ionic/amphoteric, so not the answer.
• B. Na₂O: giant ionic lattice of Na⁺ and O²⁻, so not the answer.
• C. P₄O₁₀: discrete P₄O₁₀ molecules held together by weak van der Waals forces. It is covalent but simple molecular, not giant.
• D. SiO₂: each Si atom is bonded tetrahedrally to 4 oxygen atoms, and each O is shared between 2 Si atoms. This forms a continuous network like diamond, giving a very high melting point (about 1710 °C) and extreme hardness. This is a giant covalent structure.

Step 4: Answer The correct option is D. SiO₂ (silicon(IV) oxide, the main component of sand and quartz).

Exam tip: Remember the trend across Period 3: ionic → giant covalent → simple molecular. Na₂O and MgO are ionic, SiO₂ is the one giant covalent oxide, and P₄O₁₀, SO₂ and SO₃ are simple molecular. Whenever you see "giant covalent" with oxides, think of silicon, just as carbon is the giant covalent element (diamond and graphite).