Which statement is true about crystal lattice energy?

• A. Lattice energy is always positive for ionic compounds.
• B. Lattice energy is unrelated to the stability of an ionic solid.
• C. Lattice energy increases as the charges of the ions increase.
• D. Lattice energy decreases as the size of the ions decreases.

Answer: (C)

Verified step by step guidance

1. Understand that lattice energy is the energy released when gaseous ions combine to form an ionic solid, and it is typically expressed as a negative value because energy is released (exothermic process).
2. Recall that lattice energy depends on two main factors: the charges of the ions and the distance between them (which relates to the size of the ions).
3. Use the qualitative form of Coulomb's law for lattice energy: \(E_{lattice} \propto \frac{Q_1 \times Q_2}{r}\), where \(Q_1\) and \(Q_2\) are the charges of the ions and \(r\) is the distance between ion centers (related to ionic radii).
4. Analyze the effect of increasing ion charges: since lattice energy is proportional to the product of the charges, increasing the charges increases the magnitude of lattice energy (more negative, meaning stronger attraction and more stability).
5. Analyze the effect of ionic size: decreasing the size of ions decreases the distance \(r\), which increases the magnitude of lattice energy (stronger attraction), so the statement that lattice energy decreases as ion size decreases is incorrect.