Atomic radius generally decreases across a period from left to right and increases down a group in the periodic table. Across a period, the atomic radius decreases because as the atomic number increases, protons are added to the nucleus, increasing its positive charge. Electrons are added to the same principal energy level without much increase in shielding, so the increased nuclear charge pulls the electron cloud closer to the nucleus, decreasing the size of the atom. Down a group, the atomic radius increases because while nuclear charge also increases, new electron shells (principal energy levels) are added. These outer shells are farther from the nucleus and are shielded by inner electrons, reducing the effective nuclear attraction on the outermost electrons and resulting in a larger atomic size. In summary:

• Atomic radius decreases left to right across a period due to increasing nuclear charge pulling electrons closer.
• Atomic radius increases down a group due to addition of electron shells and increased shielding despite increasing nuclear charge.