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

• A. Ionization energy increases both down a group and across a period from left to right.
• B. Ionization energy increases down a group and decreases across a period from left to right.
• C. Ionization energy decreases both down a group and across a period from left to right.
• D. Ionization energy decreases down a group and increases across a period from left to right.

Answer: (D)

Verified step by step guidance

1. Understand that ionization energy is the energy required to remove an electron from a gaseous atom or ion.
2. Recognize that as you move down a group in the periodic table, the atomic radius increases because additional electron shells are added, which causes the outer electrons to be farther from the nucleus and more shielded by inner electrons.
3. Because of this increased distance and shielding, the attraction between the nucleus and the outermost electron decreases, making it easier to remove an electron, so ionization energy decreases down a group.
4. As you move across a period from left to right, the number of protons in the nucleus increases while the electron shielding remains relatively constant, resulting in a stronger attraction between the nucleus and the outer electrons.
5. This stronger attraction makes it harder to remove an electron, so ionization energy increases across a period from left to right.