The trend towards smaller atomic radii as one moves to the right in a period is primarily due to which of the following?

• A. A decrease in electron-electron repulsion within the same shell
• B. An increase in the number of electron shells
• C. An increase in effective nuclear charge pulling electrons closer to the nucleus
• D. A decrease in the number of protons in the nucleus

Answer: (C)

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. Recognize that as you move from left to right across a period in the periodic table, the number of protons in the nucleus increases, which increases the nuclear charge.
3. Recall that electrons are added to the same principal energy level (same shell) across a period, so the shielding effect (electron-electron repulsion) does not increase significantly.
4. Realize that the increased positive charge in the nucleus (effective nuclear charge) pulls the electrons closer, reducing the atomic radius.
5. Conclude that the primary reason for the decrease in atomic radius across a period is the increase in effective nuclear charge pulling electrons closer to the nucleus.