Which of the following elements has the lowest first ionization energy?

• A. Na
• B. K
• C. Al
• D. Mg

Answer: (B)

Verified step by step guidance

1. Understand that the first ionization energy is the energy required to remove the outermost electron from a neutral atom in the gaseous state.
2. Recall the general trend in the periodic table: ionization energy increases across a period (left to right) and decreases down a group (top to bottom).
3. Identify the positions of the elements: Na (Sodium) and Mg (Magnesium) are in period 3, Al (Aluminum) is also in period 3, and K (Potassium) is in period 4 but in group 1.
4. Since K is below Na in group 1, it has a larger atomic radius and its outer electron is farther from the nucleus, experiencing less effective nuclear charge, making it easier to remove.
5. Conclude that among the given elements, K has the lowest first ionization energy because it is further down the group and its valence electron is less tightly held.