Which factor is primarily responsible for the decrease in atomic radius as you move from left to right across a period in the periodic table?

• A. Decreasing number of protons
• B. Increasing nuclear charge with constant shielding
• C. Increasing number of electron shells
• D. Decreasing effective nuclear charge

Answer: (B)

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 from left to right across a period, the number of protons in the nucleus increases, which increases the nuclear charge (positive charge attracting electrons).
3. Recognize that the electrons are being added to the same principal energy level (same shell), so the shielding effect (repulsion by inner electrons) remains approximately constant.
4. Because the nuclear charge increases while shielding remains constant, the effective nuclear charge experienced by the outer electrons increases, pulling the electrons closer to the nucleus.
5. Conclude that this increased effective nuclear charge with constant shielding is the primary factor responsible for the decrease in atomic radius across a period.