Which of the following best explains the deviations from the general trend in first ionization energy across a period?

• A. Electron configurations that result in half-filled or fully filled subshells
• B. Increasing atomic radius within a period
• C. Decreasing nuclear charge across a period
• D. Greater metallic character of elements

Answer: (A)

Verified step by step guidance

1. Understand the general trend of first ionization energy across a period: it generally increases from left to right due to increasing nuclear charge, which pulls electrons closer and makes them harder to remove.
2. Recognize that deviations from this trend occur because of electron configurations, specifically when atoms have half-filled or fully filled subshells, which are more stable and require more energy to remove an electron.
3. Recall that increasing atomic radius within a period is not correct because atomic radius actually decreases across a period, contributing to the increase in ionization energy.
4. Note that nuclear charge increases across a period, not decreases, so 'decreasing nuclear charge' cannot explain the deviations.
5. Understand that metallic character decreases across a period, so greater metallic character is not a valid explanation for deviations in ionization energy trends.