Which of the following radii of an atom is the smallest and why?
Covalent radius, crystal radius, van der Waals' radius

Explanation

To determine which of the following atomic radii is the smallest: covalent radius, crystal radius, or van der Waals radius, we can analyze each type of radius based on their definitions and the nature of the bonding involved.

### Step-by-Step Solution:

1. **Understanding Covalent Radius**:
- The covalent radius is defined as half the distance between the nuclei of two bonded atoms in a molecule. This radius is relevant for atoms that are covalently bonded, where atomic orbitals overlap to form a bond.
- Since covalent bonds involve strong interactions, the atoms are held closely together, resulting in a smaller radius.

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