Which of the following best describes the trend in ionization energy as you move down a group in the periodic table?

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

Answer: (D)

Verified step by step guidance

1. Recall that ionization energy is the energy required to remove an electron from an atom in its gaseous state.
2. Understand that as you move down a group in the periodic table, atoms have more electron shells, which increases the distance between the nucleus and the outermost electron.
3. Recognize that increased distance and additional inner electron shells cause greater electron shielding, reducing the effective nuclear charge felt by the outermost electron.
4. Since the outermost electron is held less tightly due to shielding and distance, less energy is required to remove it, meaning ionization energy decreases.
5. Therefore, the trend in ionization energy down a group is a decrease, which matches the correct answer: 'Ionization energy decreases.'