Arrange LiF, NaF, KF, RbF and CsF in order of increasing lattice energy.

Answer

To solve the problem of arranging LiF, NaF, KF, RbF, and CsF in order of increasing lattice energy, we can follow these steps:

### Step 1: Understand Lattice Energy
Lattice energy is the energy released when gaseous ions combine to form an ionic solid. It is influenced by the charges of the ions and the distance between them. The formula for lattice energy can be expressed as:
\[ U \propto \frac{Q_1 \cdot Q_2}{R} \]
where \( U \) is the lattice energy, \( Q_1 \) and \( Q_2 \) are the charges of the ions, and \( R \) is the distance between the ions.

### Step 2: Identify the Ions in Each Compound
For the compounds given:
- LiF: \( Li^+ \) and \( F^- \)
- NaF: \( Na^+ \) and \( F^- \)
- KF: \( K^+ \) and \( F^- \)
- RbF: \( Rb^+ \) and \( F^- \)
- CsF: \( Cs^+ \) and \( F^- \)

The anion \( F^- \) is the same for all compounds, and the cations are \( Li^+, Na^+, K^+, Rb^+, \) and \( Cs^+ \).

### Step 3: Compare the Sizes of the Cations
The size of the cations increases as we move down the group in the periodic table:
- \( Li^+ < Na^+ < K^+ < Rb^+ < Cs^+ \)

### Step 4: Relate Size to Lattice Energy
Since the lattice energy is inversely proportional to the distance between the ions (which is related to the size of the cations), smaller cations will lead to higher lattice energies. Therefore, the order of lattice energy will be the reverse of the order of cation size.

### Step 5: Arrange the Compounds
Based on the size of the cations, we can arrange the compounds in order of increasing lattice energy:
- CsF (largest cation, lowest lattice energy)
- RbF
- KF
- NaF
- LiF (smallest cation, highest lattice energy)

### Final Order of Increasing Lattice Energy
The final arrangement of the compounds in order of increasing lattice energy is:
\[ CsF < RbF < KF < NaF < LiF \]