How does the atomic radius change across the periodic table?

The atomic radius decreases as you move from left to right across a period on the periodic table. This phenomenon occurs due to increasing nuclear charge—that is, as you add protons to the nucleus, the positive charge of the nucleus attracts the electrons more strongly. The added electrons are being added to the same energy level and do not shield each other effectively from the increasing positive charge, leading to a greater attraction between the nucleus and the electrons. Consequently, the electrons are pulled closer to the nucleus, resulting in a smaller atomic radius.

In contrast, as you move down a group in the periodic table, the atomic radius increases despite the increase in nuclear charge because additional electron shells are added, which outweighs the effect of the increased nuclear charge. Therefore, it is essential to understand these trends in atomic structure to grasp how atomic size varies across the periodic table.