Which of the following atoms would you expect to experience a greater effective nuclear charge?

• A. Na (sodium)
• B. Ca (calcium)
• C. K (potassium)
• D. Mg (magnesium)

Answer: (D)

Verified step by step guidance

1. Understand the concept of effective nuclear charge (Z_eff), which 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 that effective nuclear charge generally increases across a period (left to right) in the periodic table because the number of protons increases while shielding remains relatively constant.
3. Note that the elements given (Na, Ca, K, and Mg) belong to different groups and periods: Na (period 3, group 1), Mg (period 3, group 2), K (period 4, group 1), and Ca (period 4, group 2).
4. Compare the effective nuclear charge experienced by valence electrons by considering both the number of protons (atomic number) and the electron shielding from inner shells. Mg has more protons than Na but is in the same period, so it experiences a higher Z_eff than Na. K and Ca are in the next period, so their valence electrons are further from the nucleus and experience more shielding, reducing their Z_eff compared to Mg.
5. Conclude that Mg experiences a greater effective nuclear charge on its valence electrons compared to Na, K, and Ca because it has a higher nuclear charge with relatively less shielding within the same period.