How does the first ionization energy change as you move down a group and across a period in the periodic table?

• A. It increases both down a group and across a period.
• B. It decreases down a group and increases across a period.
• C. It remains constant both down a group and across a period.
• D. It increases down a group and decreases across a period.

Answer: (B)

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 increases the distance between the nucleus and the outermost electron.
3. Note that the increased distance and additional electron shielding reduce the effective nuclear charge felt by the outermost electron, making it easier to remove and thus decreasing the ionization energy down a group.
4. As you move across a period from left to right, the atomic number increases, meaning more protons in the nucleus, which increases the effective nuclear charge without significantly increasing shielding because electrons are added to the same shell.
5. This stronger attraction between the nucleus and the outermost electrons makes it harder to remove an electron, so the ionization energy increases across a period.