Among the elements of the third period (Na to Ar), pick out the element
that is the most reactive non metal.

Explanation

To determine the most reactive non-metal among the elements of the third period (from sodium to argon), we can follow these steps:

### Step 1: Identify the Non-Metals in the Third Period
The elements in the third period are:
- Sodium (Na)
- Magnesium (Mg)
- Aluminium (Al)
- Silicon (Si)
...