The atomic radius of main-group elements generally increases down a group because ________.

• A. electrons are removed from the outer shell, reducing electron-electron repulsion
• B. the number of protons decreases, causing weaker attraction to electrons
• C. additional electron shells are added, increasing the distance between the nucleus and the outermost electrons
• D. the effective nuclear charge increases significantly, pulling electrons closer to the nucleus

Answer: (C)

Verified step by step guidance

1. Understand that atomic radius refers to the size of an atom, typically measured as the distance from the nucleus to the outermost electrons.
2. Recall that as you move down a group in the periodic table, elements have increasing principal quantum numbers, meaning additional electron shells are added.
3. Recognize that adding more electron shells increases the distance between the nucleus and the outermost electrons, which generally causes the atomic radius to increase.
4. Consider that although the number of protons (nuclear charge) also increases down a group, the effect of increased distance and electron shielding from inner shells reduces the effective nuclear attraction on outer electrons.
5. Conclude that the primary reason for the increase in atomic radius down a group is the addition of electron shells, which outweighs the increase in nuclear charge.