Which statement best describes the trend in ionization energy across a period and down a group, and the primary reason?

• A. Ionization energy generally increases across a period and decreases down a group; due to increasing effective nuclear charge across a period and increasing atomic size / shielding down a group.
• B. Ionization energy decreases across a period and increases down a group.
• C. Ionization energy is constant across a period.
• D. Ionization energy depends only on temperature.

Answer: (A)

Ionization energy generally rises across a period and falls down a group because of how the nucleus pulls on electrons and how distance and shielding change with the atomic structure.

Across a period, more protons are added while inner-shell shielding stays about the same, so the outer electrons feel a stronger effective nuclear charge. That stronger pull makes it harder to remove one, so ionization energy increases.

Down a group, atoms get larger and inner electrons shield the outer ones more effectively. The outer electrons are farther from the nucleus and feel a weaker pull, so less energy is needed to remove them, and ionization energy decreases.

The main reason across a period is increasing effective nuclear charge; down a group, it's increasing atomic size and shielding. There are small exceptions due to electron configuration, but the overall trend holds.