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Worked example 1

Deduce the empirical formula of the ionic compound formed between aluminium and oxygen.

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Identify the ions formed. Aluminium is in Group 13, so it loses 3 electrons to form Al³⁺. Oxygen is in Group 16, so it gains 2 electrons to form O²⁻.
Determine the ratio for neutrality. To balance the charges, we need the lowest common multiple of 3 and 2, which is 6. We need two Al³⁺ ions to get a total charge of +6 (2 x +3). We need three O²⁻ ions to get a total charge of -6 (3 x -2).
Write the formula. The ratio of Al³⁺ to O²⁻ is 2:3. Therefore, the empirical formula is Al₂O₃.

Worked example 2

Explain why magnesium oxide (MgO) has a much higher melting point (2852 °C) than sodium chloride (NaCl) (801 °C).

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Identify the ions and their charges. MgO is formed from Mg²⁺ and O²⁻ ions. NaCl is formed from Na⁺ and Cl⁻ ions.
Compare the strength of electrostatic attraction. The electrostatic attraction between ions is proportional to the product of the charges and inversely proportional to the distance between them (sum of ionic radii).
Analyse charge. The magnitude of the charges in MgO (2+ and 2-) is greater than in NaCl (1+ and 1-). This leads to a significantly stronger electrostatic force of attraction between the ions in the MgO lattice.
Analyse radii (optional but good practice). While the ionic radii also play a role (O²⁻ is similar in size to Cl⁻, Mg²⁺ is smaller than Na⁺), the dominant factor here is the doubling of charge on both ions.
Conclusion. Because the electrostatic forces of attraction between ions in the MgO lattice are much stronger than in the NaCl lattice, a great deal more energy is required to overcome these forces and break down the lattice structure. This is reflected in MgO's much larger lattice energy, and therefore, its much higher melting point.