Which of the following best describes the trend in ionization energy as you move from left to right across a period on the periodic table?

• A. Ionization energy remains constant.
• B. Ionization energy decreases.
• C. Ionization energy first increases, then decreases.
• D. Ionization energy increases.

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. Recall that as you move from left to right across a period in the periodic table, the number of protons in the nucleus increases, which increases the nuclear charge.
3. Recognize that the increased nuclear charge pulls the electrons closer to the nucleus, making it harder to remove an electron.
4. Note that the shielding effect remains relatively constant across a period because electrons are added to the same principal energy level, so the effective nuclear charge increases.
5. Conclude that because of the increasing effective nuclear charge and relatively constant shielding, the ionization energy generally increases from left to right across a period.