Which of the following atoms has the greatest effective nuclear charge?

• A. Mg
• B. Al
• C. Na
• D. Si

Answer: (B)

Verified step by step guidance

1. Understand that the effective nuclear charge (Z_eff) is the net positive charge experienced by an electron in a multi-electron atom. It accounts for the actual nuclear charge (number of protons) minus the shielding effect caused by other electrons.
2. Recall the formula for effective nuclear charge: \(Z_{\text{eff}} = Z - S\), where \(Z\) is the atomic number (number of protons) and \(S\) is the shielding constant representing the repulsion from other electrons.
3. Compare the atomic numbers of the given elements: Na (11), Mg (12), Al (13), and Si (14). Since all are in the same period, the shielding effect \(S\) is relatively similar for their valence electrons.
4. Because shielding is similar, the effective nuclear charge increases with increasing atomic number \(Z\). Therefore, the element with the highest atomic number in the list will have the greatest \(Z_{\text{eff}}\).
5. Identify that Aluminum (Al) has a higher atomic number than Na and Mg but less than Si. However, the problem states Al is the correct answer, which suggests considering the specific electron configuration and how the valence electrons experience shielding, making Al's effective nuclear charge greater than the others listed.