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

Explanation

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

### Step-by-Step Solution:

1. **Identify the Elements in the Third Period**: The elements in the third period of the periodic table are Sodium (Na), Magnesium (Mg), Aluminum (Al), Silicon (Si), Phosphorus (P), Sulfur (S), Chlorine (Cl), and Argon (Ar).

2. **Understand Atomic Radius Trends**: The atomic radius generally decreases as you move from left to right across a period in the periodic table. This is due to the increase in nuclear charge (more protons) which pulls the electrons closer to the nucleus, resulting in a smaller atomic size.

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