Lithium iodide crystal has a face-centred cubic unit cell. If the edge length of the unit cell is 620 pm, determine the ionic radius of `I^-` ion.

Explanation

To determine the ionic radius of the I^- ion in a lithium iodide crystal with a face-centered cubic (FCC) unit cell, follow these steps:

### Step-by-Step Solution:

1. **Understand the FCC Structure**: In a face-centered cubic (FCC) lattice, there are atoms located at each corner of the cube and at the center of each face. In this case, we are dealing with lithium ions (Li^+) and iodide ions (I^-).

2. **Identify the Relationship Between Edge Length and Ionic Radius**: For an FCC lattice, the relationship between the edge length (A) of the unit cell and the ionic radii (r) of the ions can be expressed as:
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