Which factor would be most likely to decrease the size of an atom's electron cloud?

• A. Decreasing the number of protons in the nucleus
• B. Adding electrons to the outermost shell
• C. Increasing electron shielding
• D. Increasing the effective nuclear charge (Z_eff)

Answer: (D)

Verified step by step guidance

1. Understand that the size of an atom's electron cloud is influenced by the effective nuclear charge (\(Z_{eff}\)), which is the net positive charge experienced by electrons after accounting for shielding by other electrons.
2. Recognize that increasing the number of protons in the nucleus increases \(Z_{eff}\), pulling electrons closer and decreasing the size of the electron cloud.
3. Note that decreasing the number of protons reduces \(Z_{eff}\), allowing the electron cloud to expand due to weaker attraction.
4. Adding electrons to the outermost shell increases electron-electron repulsion and shielding, which generally decreases \(Z_{eff}\) and causes the electron cloud to expand.
5. Increasing electron shielding reduces \(Z_{eff}\) by blocking the nuclear charge, leading to a larger electron cloud, so the factor that decreases the size is actually increasing \(Z_{eff}\).