Which of the following alkali metal halides has the lowest lattice energy

Options

A. LiF
B. NaCl
C. KBr
D. Csl

Answer

To determine which alkali metal halide has the lowest lattice energy, we need to analyze the factors that influence lattice energy. Lattice energy is the energy released when gaseous ions combine to form an ionic solid. It is influenced by the size of the ions involved and the charge of the ions.

### Step-by-Step Solution:

1. **Understand Lattice Energy**: Lattice energy is inversely related to the size of the ions. Larger ions lead to lower lattice energy because the distance between the ions increases, reducing the electrostatic attraction.

2. **Identify the Alkali Metal Halides**: The alkali metal halides typically include compounds formed from alkali metals (Li, Na, K, Rb, Cs) and halogens (F, Cl, Br, I). For this question, we will consider the halides of alkali metals like LiF, NaCl, KBr, and CsI.

3. **Compare Ion Sizes**:
- **Cations**: The size of alkali metal cations increases down the group: Li+ < Na+ < K+ < Rb+ < Cs+.
- **Anions**: The size of halide anions increases down the group: F- < Cl- < Br- < I-.

4. **Analyze CsI**: In the case of CsI, both Cs+ (the largest alkali metal cation) and I- (the largest halide anion) are present. This results in a larger distance between the ions compared to other alkali metal halides.

5. **Conclusion**: Since CsI has the largest ions among the options, it will have the lowest lattice energy due to the weaker electrostatic attraction between the larger cation and anion.

### Final Answer:
The alkali metal halide with the lowest lattice energy is **CsI**.

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