As electrons are added to a shell within the same principal energy level, how does the atomic radius typically change?

• A. The atomic radius increases because more electrons repel each other.
• B. The atomic radius decreases due to increased effective nuclear charge.
• C. The atomic radius remains constant since the principal energy level does not change.
• D. The atomic radius fluctuates unpredictably with each added electron.

Answer: (B)

Verified step by step guidance

1. Understand that the atomic radius is influenced by the balance between the attraction of the nucleus for the electrons and the repulsion among electrons themselves.
2. Recognize that within the same principal energy level (same shell), adding electrons increases the number of negatively charged particles, but the number of protons in the nucleus remains constant.
3. Recall the concept of effective nuclear charge (\(Z_{eff}\)), which is the net positive charge experienced by an electron after accounting for shielding by other electrons.
4. As more electrons are added to the same shell, the shielding effect does not increase significantly because these electrons are in the same energy level, so the effective nuclear charge felt by each electron actually increases.
5. An increased effective nuclear charge pulls the electrons closer to the nucleus, causing the atomic radius to decrease as electrons are added within the same principal energy level.