Why do both atomic size and ionic size increase as you move down a group in the periodic table?

• A. Because the effective nuclear charge increases significantly, pulling electrons closer to the nucleus.
• B. Because additional electron shells are added, increasing the distance between the nucleus and the outermost electrons.
• C. Because atoms lose protons as you move down a group, reducing the attraction to electrons.
• D. Because the number of valence electrons decreases, causing the atom to expand.

Answer: (B)

Verified step by step guidance

1. Understand that atomic size and ionic size refer to the radius of an atom or ion, which depends on the distance between the nucleus and the outermost electrons.
2. Recognize 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. Know that adding more electron shells increases the distance between the nucleus and the outermost electrons, which generally causes the size of the atom or ion to increase.
4. Consider the concept of effective nuclear charge (Z_eff), which is the net positive charge experienced by valence electrons after accounting for shielding by inner electrons; although Z_eff increases slightly down a group, the effect of added shells dominates.
5. Conclude that the primary reason atomic and ionic sizes increase down a group is the addition of electron shells, which outweighs the modest increase in effective nuclear charge, leading to a larger radius.