Which of the following has the highest lattice energy ?

Options

A. LiBr
B. LiCl
C. LiI
D. LiF

Answer

To determine which ionic compound has the highest lattice energy, we can follow these steps:

### Step 1: Understand Lattice Energy
Lattice energy is defined as the amount of energy released when one mole of an ionic compound is formed from its gaseous ions. It is influenced by the charges of the ions and the distance between them.

### Step 2: Lattice Energy Formula
The lattice energy (U) can be expressed as:
\[ U \propto \frac{Q_1 \cdot Q_2}{R} \]
Where:
- \( Q_1 \) and \( Q_2 \) are the charges of the cation and anion, respectively.
- \( R \) is the distance between the ions (the ionic radii).

### Step 3: Analyze the Charges
For the options provided (LiF, NaCl, KBr, etc.), we need to consider the charges of the ions. In most common ionic compounds:
- LiF: \( Li^+ \) and \( F^- \) (charges: +1 and -1)
- NaCl: \( Na^+ \) and \( Cl^- \) (charges: +1 and -1)
- KBr: \( K^+ \) and \( Br^- \) (charges: +1 and -1)

All these compounds have ions with the same charges, which is +1 and -1.

### Step 4: Consider Ionic Radii
The next factor to consider is the ionic radii. As we move down a group in the periodic table, the size of the ions increases due to the addition of electron shells. Therefore:
- \( Li^+ \) is smaller than \( Na^+ \) and \( K^+ \).
- \( F^- \) is smaller than \( Cl^- \) and \( Br^- \).

### Step 5: Determine the Compound with the Smallest Ions
Since lattice energy is inversely proportional to the distance (ionic radii), the compound with the smallest ions will have the highest lattice energy. In this case:
- Lithium fluoride (LiF) has the smallest ions compared to sodium chloride (NaCl), potassium bromide (KBr), and other options.

### Conclusion
Thus, the compound with the highest lattice energy is **Lithium Fluoride (LiF)**.