What is atomic radius? Why does atomic radius decrease across a period?

Explanation

### Step-by-Step Solution:

**Step 1: Define Atomic Radius**
Atomic radius is defined as the distance from the nucleus of an atom to the outermost shell of electrons, which is also referred to as the valence shell. It gives an idea of the size of the atom.

**Step 2: Understand the Periodic Table Structure**
The periodic table is organized into horizontal rows called periods and vertical columns called groups. There are 7 periods and 18 groups in the modern periodic table.

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