Express the relation between atomic radius (r) and edge length (a) in b.c.c. unit cell.

Explanation

To derive the relation between the atomic radius (r) and the edge length (a) in a body-centered cubic (BCC) unit cell, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Structure of BCC**:
In a BCC unit cell, there are atoms located at each of the eight corners of the cube and one atom at the center of the cube.

2. **Identify the Body Diagonal**:
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