How does atomic size change across and down the periodic table?

Atomic size follows a consistent pattern across the periodic table, generally increasing from the top to the bottom of a group and decreasing from left to right across a period.12

Trends Down a Group

As you move down a group, atomic radius increases. This occurs because each successive element adds electrons to a new, outer energy shell, effectively creating a larger layer of the atomic structure.1

Trends Across a Period

Moving from left to right across a period, atomic size decreases. Although these atoms contain more electrons, they also possess a higher nuclear charge due to an increasing number of protons. Because inner-shell electrons provide ineffective shielding for the outer valence electrons, the stronger positive charge of the nucleus pulls the outer electrons closer, resulting in a more compact atom.1

Consequently, the largest atoms are located at the lower left of the periodic table, while the smallest atoms are found at the upper right.12