Atomic Radius: the distance from the nucleus of one atom to the electrons on its valence shell. (Hint: think of a circle and its radius—the atom has the same shape).
You can also think of AR as the distance from one atom to the next.
Trend: As you proceed from:
RIGHT → LEFT AR ↑
TOP → BOTTOM AR ↑
Why?
From right to left, the (Zeff) ↓ and electrons are not pulled in with the same force, allowing them to roam further out, extending the AR.
From top to bottom, the “n” or shells increase, thereby increasing the space between the nucleus and the valence shell, also extending the AR.
ChemistryBytes.com 