How do atomic radius vary in a period and in a group? How do you explain the variation?
The atomic radii generally decrease across a period from left to right. Conversely, within a group, the atomic radius increases regularly as we move down with increasing atomic number.
Across a period, the outer electrons are in the same valence shell. As the atomic number increases, the effective nuclear charge increases, resulting in an increased attraction of electrons to the nucleus, causing the atomic size to decrease.
Down a group, the principal quantum number () increases, meaning the valence electrons are farther from the nucleus. Additionally, the inner energy levels are filled with electrons, which serve to shield the outer electrons from the pull of the nucleus, leading to an increase in atomic radius.
Explanation
The answer explains the two distinct trends of atomic radius. In a period, the atomic size decreases due to increasing effective nuclear charge while electrons are added to the same valence shell. In a group, the atomic size increases because the principal quantum number increases and inner electron shells shield the outer electrons from the nuclear pull.