Which statement describes the ionization energy trend across the periodic table?

Which statement describes the ionization energy trend across the periodic table?

• A. Increases down a group and increases across a period
• B. Decreases down a group and increases across a period
• C. Increases down a group and decreases across a period
• D. Decreases down a group and decreases across a period

Answer: (B)

The question tests how the energy required to remove the outermost electron (first ionization energy) changes across the periodic table. As you move across a period from left to right, the nucleus gains more protons while electrons are added to the same energy level. This increases the effective nuclear charge felt by the outer electrons, pulling them closer and making them harder to remove. So ionization energy increases across a period. As you move down a group, you add successive electron shells. The outer electrons sit farther from the nucleus and are more shielded by inner electrons, so the attraction between the nucleus and the outer electron weakens. This makes it easier to remove an electron, so ionization energy decreases down a group. Putting it together, ionization energy decreases down a group and increases across a period, which is why that statement is correct.

The question tests how the energy required to remove the outermost electron (first ionization energy) changes across the periodic table.

As you move across a period from left to right, the nucleus gains more protons while electrons are added to the same energy level. This increases the effective nuclear charge felt by the outer electrons, pulling them closer and making them harder to remove. So ionization energy increases across a period.

As you move down a group, you add successive electron shells. The outer electrons sit farther from the nucleus and are more shielded by inner electrons, so the attraction between the nucleus and the outer electron weakens. This makes it easier to remove an electron, so ionization energy decreases down a group.

Putting it together, ionization energy decreases down a group and increases across a period, which is why that statement is correct.