Why do atomic radii decrease as you move from left to right across a period in the periodic table?

• A. Because atoms lose electrons as you move across a period, making them smaller.
• B. Because the shielding effect increases significantly, causing the atomic radius to decrease.
• C. Because the number of protons increases, resulting in a stronger effective nuclear charge that pulls electrons closer to the nucleus.
• D. Because additional electron shells are added, increasing the distance between the nucleus and outer electrons.

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 electron cloud.
2. Recognize that as you move from left to right across a period, the number of protons in the nucleus increases, which increases the positive charge of the nucleus.
3. Know that electrons are added to the same principal energy level (same shell) across a period, so the shielding effect (repulsion by inner electrons) remains relatively constant.
4. Because the effective nuclear charge (\(Z_{eff}\)) felt by the outer electrons increases due to the higher number of protons without a significant increase in shielding, the nucleus pulls the electrons closer.
5. This stronger attraction causes the atomic radius to decrease across the period, as electrons are held more tightly and the atom becomes smaller.