Which statement best describes the trend of atomic radius across the periodic table?

• A. Increases down a group; Decreases across a period
• B. Decreases down a group; Increases across a period
• C. Both A and B
• D. Remains constant across groups and periods

Answer: (C)

Atomic radius reflects how far the outermost electrons are from the nucleus, shaped by shielding and effective nuclear charge. As you move down a group, each step adds a new electron shell, so outer electrons sit farther from the nucleus and shielding weakens the pull of the nucleus; the radius grows. Across a period, electrons enter the same shell while the number of protons increases, pulling the outer electrons in more tightly, so the radius shrinks. Therefore the radius increases down a group and decreases across a period, and the description that combines both of these parts is the correct one. The idea that it stays constant or that the two trends are reversed doesn’t fit the observed pattern.