Define (a) metallic radius and (b) van der Waals radius.

Explanation

To define the terms "metallic radius" and "van der Waals radius," we can break down the definitions step by step.

### Step-by-Step Solution:

**(a) Metallic Radius:**
1. **Understanding the Concept**: The metallic radius refers to the distance between the nuclei of two adjacent metal atoms that are bonded together in a metallic structure.
2. **Distance Measurement**: When two metal atoms bond, the distance between their nuclei can be measured.
3. **Calculating the Radius**: The metallic radius is defined as half of the distance between the centers of the nuclei of these two bonded atoms.
...