Among the elements of the third period (Na to Ar), pick out the element
with the largest atomic radius.

Explanation

To determine which element in the third period (from sodium to argon) has the largest atomic radius, we can follow these steps:

### Step 1: List the Elements
The elements in the third period are:
- Sodium (Na)
- Magnesium (Mg)
- Aluminum (Al)
- Silicon (Si)
...