In the periodic table, where is atomic size generally larger?

• A. Left and down
• B. Right and up
• C. Left and up
• D. Right and down

Answer: (A)

Atomic size tracks how many electron shells an atom has and how strongly the nucleus pulls on the outer electrons. As you move down a group, you add a new electron shell, and shielding by inner electrons grows, so outer electrons sit farther from the nucleus and the atomic radius increases. Across a period from left to right, the nucleus pulls more strongly on the same set of electrons (increasing effective nuclear charge) while shielding changes little, so the outer electrons are held more tightly and the radius decreases. Because of this, atoms are generally larger on the left side and toward the bottom of the table.