Why does ionization energy tend to increase as you move across a period in the periodic table?

• A. Because the atomic radius increases, making it harder to remove an electron.
• B. Because the number of valence electrons decreases, making electrons more tightly held.
• C. Because the number of energy levels increases, increasing electron shielding.
• D. Because the nuclear charge increases, causing a stronger attraction between the nucleus and valence electrons.

Answer: (D)

Verified step by step guidance

1. Understand that ionization energy is the energy required to remove an electron from an atom in its gaseous state.
2. Recognize that as you move across a period from left to right, the number of protons in the nucleus (nuclear charge) increases, while electrons are added to the same principal energy level.
3. Note that the increase in nuclear charge results in a stronger electrostatic attraction between the positively charged nucleus and the negatively charged valence electrons.
4. Since electrons are added to the same energy level, the shielding effect (repulsion from inner electrons) does not increase significantly, so it does not offset the increased nuclear charge.
5. Therefore, the stronger attraction due to increased nuclear charge makes it more difficult to remove an electron, causing the ionization energy to increase across a period.