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

• A. Aluminum (Al)
• B. Boron (B)
• C. Indium (In)
• D. Gallium (Ga)

Answer: (B)

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 in the periodic table: Boron (B) and Aluminum (Al) are in group 13, with Boron in period 2 and Aluminum in period 3; Gallium (Ga) and Indium (In) are also in group 13 but in periods 4 and 5 respectively.
3. Understand that as you move down a group, the atomic radius increases due to additional electron shells, which causes the outer electrons to be farther from the nucleus and less tightly held, thus lowering the ionization energy.
4. Recognize that Boron, being at the top of group 13, has fewer electron shells and a smaller atomic radius, so its outer electron is held more tightly, resulting in a higher first ionization energy compared to Aluminum, Gallium, and Indium.
5. Conclude that among the given elements, Boron has the highest first ionization energy because it is the smallest atom in the group and its valence electron is most strongly attracted to the nucleus.