For Group 5A elements, the ionization energy decreases going down a group because:

• A. Their atomic masses increases.
• B. The highest number of their valence orbital shell increases.
• C. Their metallic character decreases.
• D. Their electron affinity increases.
• E. The number of neutrons in their nuclei decreases.

Answer: (B)

Verified step by step guidance

1. Understand that ionization energy is the energy required to remove an electron from an atom in the gaseous state.
2. Recognize that as you move down a group in the periodic table, the principal quantum number (n) of the valence electrons increases, meaning electrons are in higher energy levels further from the nucleus.
3. Realize that with each step down the group, an additional electron shell is added, increasing the distance between the nucleus and the valence electrons, which reduces the effective nuclear charge experienced by these electrons.
4. Acknowledge that the increased distance and additional electron shielding reduce the attraction between the nucleus and the valence electrons, making it easier to remove an electron, thus decreasing ionization energy.
5. Conclude that the correct reason for the decrease in ionization energy down Group 5A is the increase in the principal quantum number of the valence shell, not the atomic mass or other factors listed.