1. Define the term “isotopes” and state two differences between protium and deuterium. (3 marks)

題解： Isotopes are atoms of the same element with the same atomic number but different mass numbers. Protium has no neutron while deuterium has one neutron. Protium has mass number 1, deuterium has mass number 2.

2. Draw the dot-and-cross diagram of magnesium oxide and explain why it has high melting point. (4 marks)

題解： MgO consists of Mg²⁺ and O²⁻ ions held by strong electrostatic forces in a giant ionic lattice. Large amount of energy is required to break the strong ionic bonds, hence high melting point.

3. Calculate the number of moles in 11.2 dm³ of CO₂ at room temperature and pressure. (2 marks)

題解： At r.t.p., 1 mole of gas occupies 24 dm³. Moles of CO₂ = 11.2 ÷ 24 = 0.467 mol.

4. Explain the trend of atomic radius across Period 3. (3 marks)

題解： Atomic radius decreases across Period 3 because the number of protons increases while shielding effect remains similar. Effective nuclear charge increases, pulling electrons closer to the nucleus.

5. Write the balanced equation for the reaction between sodium and water. (2 marks)

題解： 2Na + 2H₂O → 2NaOH + H₂