Which of the following has greatest lattice energy? MgBr2 MgF2 MgCl2 MgI
The Correct Answer and Explanation is:
The correct answer is: MgF₂
Explanation:
Lattice energy is the energy released when one mole of an ionic crystalline compound is formed from its gaseous ions. It is a measure of the strength of the forces holding the ions together in an ionic solid.
Factors Affecting Lattice Energy:
- Charge of the ions
Greater charges on the ions result in a stronger electrostatic attraction and higher lattice energy. - Size (radius) of the ions
Smaller ions can pack more closely, resulting in stronger attraction between the oppositely charged ions and thus higher lattice energy.
All the compounds listed — MgF₂, MgCl₂, MgBr₂, and MgI₂ — contain the same cation, Mg²⁺, which has a +2 charge. So, to determine the compound with the greatest lattice energy, we compare the anions:
- F⁻ (fluoride): smallest radius
- Cl⁻ (chloride): larger than F⁻
- Br⁻ (bromide): larger than Cl⁻
- I⁻ (iodide): largest
Because F⁻ is the smallest anion, it allows the Mg²⁺ cation to come closer, which increases the electrostatic force and results in a greater lattice energy. As the anion size increases from F⁻ to I⁻, the distance between the centers of the ions increases, reducing the lattice energy.
Ranking of lattice energy (from greatest to least):
- MgF₂ (smallest anion, highest lattice energy)
- MgCl₂
- MgBr₂
- MgI₂ (largest anion, lowest lattice energy)
Therefore, MgF₂ has the greatest lattice energy due to the small size of the fluoride ion and the strong attraction it forms with the Mg²⁺ ion.
