Which of the following best explains why atomic radius increases as you move down a group in the periodic table?

• A. The number of protons decreases, reducing the effective nuclear charge.
• B. Electrons are removed from the atom, causing the atom to expand.
• C. Additional electron shells are added, increasing the distance between the nucleus and the outermost electrons.
• D. Atoms gain more neutrons, which increases the size of the electron cloud.

Answer: (C)

Verified step by step guidance

1. Understand that atomic radius refers to the size of an atom, typically measured from the nucleus to the outer boundary of the electron cloud.
2. Recall that as you move down a group in the periodic table, each successive element has an additional electron shell compared to the one above it.
3. Recognize that adding more electron shells increases the distance between the nucleus and the outermost electrons, which leads to a larger atomic radius.
4. Consider the concept of effective nuclear charge (Z_eff), which is the net positive charge experienced by outer electrons; although the number of protons increases down a group, the shielding effect of inner shells reduces the pull on outer electrons.
5. Conclude that the primary reason atomic radius increases down a group is the addition of electron shells, which outweighs the effect of increased nuclear charge, causing the atom to expand.