What does the term "shielding" in the context of ionization energy refer to?

The term "shielding" in the context of ionization energy specifically refers to the reduction in attraction between the nucleus and outer electrons. As electrons occupy different energy levels or shells around the nucleus, the inner-shell electrons repel the outer electrons due to their like charge. This repulsion effectively "shields" the outer electrons from the full positive charge of the nucleus. When the outer electrons experience this reduced effective nuclear charge, it becomes easier to remove them, thereby decreasing the ionization energy required to detach these electrons.

Understanding shielding is crucial because as one moves down a group in the periodic table, the number of inner electron shells increases, leading to greater shielding. Consequently, the ionization energy decreases because the effective nuclear charge felt by the outermost electrons diminishes.