Answer
The cation-anion distance is estimated to be as follows:
$Li-F: 2.09$ angstrom
$Na-Cl: 2.83$ angstrom
$K-Br: 3.34$ angstrom
$Rb-I:3.72$ angstrom
Work Step by Step
The ionic radii of $Li^+$ is $0.90$ angstrom.
The ionic radii of $F^-$ is $1.19$ angstrom.
The ionic radii of $Na^+$ is $1.16$ angstrom.
The ionic radii of $Cl^-$ is $1.67$ angstrom.
The ionic radii of $K^+$ is $1.52$ angstrom.
The ionic radii of $Br^-$ is $1.82$ angstrom.
The ionic radii of $Rb^+$ is $1.66$ angstrom.
The ionic radii of $I^-$ is $2.06$ angstrom.
Now, to estimate the cation-anion distance, we would take the sum of the ionic radii of 2 ions in the ionic compound. In detail,
- $Li-F:0.90+1.19=2.09$ angstrom
- $Na-Cl:1.16+1.67=2.83$ angstrom
- $K-Br:1.52+1.82=3.34$ angstrom
- $Rb-I:1.66+2.06=3.72$ angstrom
