Which of the following best describes the periodic trend for ionization energy across the periodic table?

• A. Ionization energy remains constant across periods and groups.
• B. Ionization energy increases from left to right across a period and decreases from top to bottom down a group.
• C. Ionization energy decreases from left to right across a period and decreases from top to bottom down a group.
• D. Ionization energy decreases from left to right across a period and increases from top to bottom down a group.

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. Recall that across a period (left to right), the nuclear charge increases while the shielding effect remains relatively constant, causing electrons to be held more tightly.
3. Recognize that because electrons are held more tightly across a period, ionization energy generally increases from left to right.
4. Consider that down a group (top to bottom), the atomic radius increases due to additional electron shells, which increases shielding and reduces the effective nuclear charge felt by outer electrons.
5. Therefore, ionization energy decreases down a group because outer electrons are held less tightly and are easier to remove.