Advanced Placement (AP) Chemistry Practice Exam 2025 - Free AP Chemistry Practice Questions and Study Guide

Shielding electrons are specifically the electrons located between the nucleus of an atom and its valence electrons. Their primary role is to create a physical barrier that reduces the effective nuclear charge experienced by the valence electrons. This occurs because the shielding electrons repel the valence electrons and lessen the full attractive force that the positively charged protons in the nucleus exert on these outer electrons. As a result, the outermost electrons feel a diminished attraction to the nucleus, which influences various atomic properties, including atomic size and ionization energy.

The other options do not accurately describe shielding electrons. The first option pertains to factors affecting atomic radius but does not define the concept of shielding. The second option refers to electrons in the outermost shell, which may or may not be involved in shielding depending on the context of the atom. The last option mentions ionization energy, which is affected by shielding but does not define shielding electrons themselves. Thus, option C correctly captures the essence of shielding electrons and their function in an atom.