Which of the following elements has the smallest first ionization energy (IE\(_1\))?

• A. Mg
• B. Na
• C. Al
• D. K

Answer: (D)

Verified step by step guidance

1. Recall that the first ionization energy (IE\(_1\)) is the energy required to remove the outermost electron from a neutral atom in the gas phase.
2. Understand the periodic trends: Ionization energy generally increases across a period (left to right) due to increasing nuclear charge and decreases down a group because the outer electrons are farther from the nucleus and more shielded.
3. Identify the positions of the given elements (Mg, Na, Al) and the correct answer (K) on the periodic table: Mg, Na, and Al are in the third period, while K is in the fourth period and is an alkali metal.
4. Compare the elements: Since K is in the fourth period and an alkali metal, it has its outermost electron in a higher energy level (4s) and experiences more shielding, making it easier to remove than the others in the third period.
5. Conclude that potassium (K) has the smallest first ionization energy among the listed elements because it loses its outer electron more easily due to its position in the periodic table.