Which of the following elements has the smallest ionization energy?

• A. K
• B. Al
• C. Na
• D. Mg

Answer: (A)

Verified step by step guidance

1. Recall that ionization energy is the energy required to remove an electron from a gaseous atom or ion. Generally, ionization energy increases across a period (left to right) and decreases down a group (top to bottom) in the periodic table.
2. Identify the positions of the given elements in the periodic table: Aluminum (Al), Sodium (Na), Magnesium (Mg), and Potassium (K). Note their group and period numbers.
3. Compare the elements based on their positions: K is in Group 1 and Period 4, Na is in Group 1 and Period 3, Mg is in Group 2 and Period 3, and Al is in Group 13 and Period 3.
4. Since ionization energy decreases down a group, potassium (K), being below sodium (Na) in Group 1, will have a lower ionization energy than Na. Also, elements in Group 1 generally have lower ionization energies than those in Groups 2 and 13 in the same period.
5. Conclude that potassium (K) has the smallest ionization energy among the given elements because it is the lowest in Group 1 and ionization energy decreases down the group.