In an `LiI` crystal, `I^(-)` ions from a cubical close-packed arrangement, and `Li^(+)` ion occupy octahedral holes. What is the relationship between the edge length of the unit cells and radii of the `I^(-)` ions if `a = 60` pm.

Explanation

To find the relationship between the edge length of the unit cell (a) and the radii of the ions (R⁻ for I⁻ ions and R⁺ for Li⁺ ions) in an LiI crystal, we can follow these steps:

### Step 1: Understand the arrangement of ions
In the LiI crystal, the I⁻ ions form a cubic close-packed (CCP) arrangement, which corresponds to a face-centered cubic (FCC) lattice. The Li⁺ ions occupy the octahedral holes in this arrangement.

### Step 2: Analyze the unit cell
In a face-centered cubic unit cell:
- There are 4 I⁻ ions per unit cell (1/8 from each of the 8 corners and 1/2 from each of the 6 faces).
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