AP Chemistry Practice Exam Prep: Comprehensive Practice Test & Study Guide

The increasing effective nuclear charge across a period is largely explained by the concept of effective nuclear charge itself, which signifies the net positive charge experienced by an electron in a multi-electron atom. As protons are added to the nucleus when moving from left to right across a period, the number of protons increases, resulting in a stronger attraction between the nucleus and the electrons.

While electrons are also added to the same energy level, they do not fully shield each other from the positive charge of the nucleus. This inadequacy of shielding leads to a situation where the pull from the increasing number of protons outweighs the slight increase in electron-electron repulsion, enhancing the effective nuclear charge that valence electrons experience.

Understanding that shielding plays a key role, it becomes clear why effective nuclear charge increases across a period. Electrons in the outermost shell are drawn closer to the nucleus due to this stronger attraction, which contributes to the trends seen in atomic size and ionization energy across periods.