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

• A. Mg
• B. Al
• C. K
• D. Na

Answer: (C)

Verified step by step guidance

1. Recall that the first ionization energy is the energy required to remove one mole of electrons from one mole of gaseous atoms, and it generally 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 elements Mg, Al, K, and Na on the periodic table: Mg and Al are in period 3, with Al to the right of Mg; Na and K are in groups 1 but K is in period 4, below Na.
3. Understand that elements in group 1 (alkali metals) have lower ionization energies compared to elements in groups 2 and 13 because they have a single electron in their outermost shell that is easier to remove.
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, which lowers its first ionization energy compared to Na, Mg, and Al.
5. Conclude that among the given elements, K has the smallest first ionization energy because it is an alkali metal located further down the group, making it easier to remove its outermost electron.