Which of the following statements best explains the trend in ionization potential across period 2 of the periodic table, particularly focusing on the role of subshells and electron affinity?

Across period 2 the ionization potential rises from lithium to neon because each successive element adds a proton while the valence electrons remain in the same principal shell, so the outer electrons feel a stronger effective nuclear charge. This increased attraction pulls the electrons closer and makes them harder to remove. The trend also reflects how the 2s and 2p subshells fill: once the 2s subshell is full, the next electrons enter the 2p subshell, where the added electrons experience both the higher nuclear charge and the repulsion from the already‑filled 2s electrons, raising the ionization energy. For example, removing an electron from magnesium (which has a filled 2s subshell) requires more energy than from sodium, whose valence electron occupies the 2p subshell and is less tightly bound. Thus, the interplay of increasing nuclear charge, subshell filling, and electron affinity explains the steady rise in ionization potential across period 2.