Atomic Radius
How large an atom is, measured here as van der Waals radius — the effective size an atom occupies when it isn't chemically bonded to anything.
See atomic radius colored on the interactive table →Across a period (left → right)
Shrinks as you move left to right across a period.
Down a group (top → bottom)
Grows as you move down a group.
Why it happens
Moving across a period, each element adds a proton to the nucleus and an electron to the *same* outer shell. The extra nuclear charge pulls that whole shell in tighter, so the atom actually gets smaller even though it has more electrons. Moving down a group, each new element adds an entire new electron shell further from the nucleus, and that added distance outweighs the stronger pull from the larger nucleus — so atoms get bigger.