Which of the following correctly describes how ionization energy changes across a period in the periodic table?

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

Answer: (C)

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 (from left to right) in the periodic table, the nuclear charge increases because more protons are added to the nucleus.
3. Recognize that as the nuclear charge increases, the attraction between the nucleus and the electrons also increases, making it harder to remove an electron.
4. Note that the atomic radius generally decreases across a period, which means electrons are held more tightly by the nucleus, further increasing ionization energy.
5. Conclude that due to the increasing nuclear charge and decreasing atomic radius, ionization energy generally increases across a period.