Atomic radius trends in period 3

Steps:

• Period 3 elements range from sodium (left) to argon (right), excluding argon for X and Y.
• Atomic radius decreases across the period due to increasing effective nuclear charge pulling electrons closer.
• X's greater radius places it left of Y in the period.
• Left-side elements are metals; right-side are non-metals, so Y is a non-metal.

Why D is correct:

• In period 3, non-metals occupy positions to the right (higher atomic number), where atomic radii are smaller due to stronger nuclear attraction.

Why the others are wrong:

• A: X (left) has fewer protons than Y (right), as atomic number increases across the period.
• B: Elements in the same period have the same number of electron shells, thus equal total electrons for neutral atoms? No, atomic number determines electrons, increasing left to right.
• C: Outer (valence) electrons increase from 1 (group 1) to 7 (group 17) across period 3.

Final answer: D