Which of the following best explains the general trends and exceptions observed in ionization energy across the periodic table?

• A. Ionization energy trends are determined only by the number of protons in the nucleus.
• B. Ionization energy increases solely because atomic radius decreases, with no exceptions.
• C. Exceptions to ionization energy trends are caused by changes in temperature and pressure.
• D. The effective nuclear charge and electron shielding influence ionization energy, while exceptions arise due to electron configurations such as filled and half-filled subshells.

Answer: (D)

Verified step by step guidance

1. Understand that ionization energy is the energy required to remove an electron from an atom in its gaseous state.
2. Recognize that ionization energy generally increases across a period (left to right) because the effective nuclear charge (Z_eff) increases, pulling electrons closer and making them harder to remove.
3. Note that ionization energy generally decreases down a group (top to bottom) because electrons are added to higher energy levels farther from the nucleus, increasing atomic radius and electron shielding, which reduces the effective nuclear charge felt by outer electrons.
4. Identify that exceptions to these trends occur due to specific electron configurations, such as filled or half-filled subshells, which provide extra stability and thus affect the ionization energy values.
5. Conclude that ionization energy trends are influenced by a combination of effective nuclear charge, electron shielding, atomic radius, and electron configuration, rather than by a single factor like proton number or atomic radius alone.