MCAT Question A Day - 4/8/13 - Answer!
Reading from top to bottom down a column of the Periodic Table of Elements, the atomic radius becomes
A. larger, owing to increased nuclear charge that is only partially shielded by additional valence electrons.
B. smaller, owing to increased nuclear charge that is only partially shielded by additional valence electrons.
C. smaller, owing to placement of additional electrons in orbitals that are farther from the nucleus.
D. larger, owing to placement of additional electrons in orbitals that are farther from the nucleus.
The correct answer is (D). Choices (A) and (B) could be used to explain variation in radius across a row, but that is not asked for here. As one moves down a column, the highest-filled orbital becomes farther from the nucleus, causing the atomic radii to be greater; e.g., the radius of K, whose highest-filled orbital is 4s, is greater than Na, whose highest-filled orbital is 3s.
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