Sodium crystallises in a body-centred cubic unit cell. (bcc) with edge length `4.29Å`. What is the radius of the sodium atom ? What is the length of the body-diagonal of the unit cell?

Explanation

To solve the problem step by step, we will find the radius of the sodium atom and the length of the body diagonal of the body-centered cubic (BCC) unit cell.

### Step 1: Understand the BCC structure
In a body-centered cubic (BCC) unit cell, there are atoms located at each corner of the cube and one atom at the center of the cube. The relationship between the edge length (A) and the radius (r) of the atom in a BCC structure is given by the formula:

\[ 4r = \sqrt{3}A \]
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