From left to right across a period for main-group elements, how does the effective nuclear charge (Z_{eff}) change?

• A. It increases.
• B. It decreases.
• C. It remains constant.
• D. It first increases, then decreases.

Answer: (A)

Verified step by step guidance

1. Understand that effective nuclear charge (\(Z_{\text{eff}}\)) is the net positive charge experienced by an electron in a multi-electron atom. It accounts for the actual nuclear charge (\(Z\)) minus the shielding effect caused by other electrons.
2. Recall that as you move from left to right across a period in the periodic table, the atomic number (\(Z\)) increases because more protons are added to the nucleus.
3. Note that electrons are added to the same principal energy level (same shell) across a period, so the shielding effect does not increase significantly because inner electron shells remain the same.
4. Since the shielding remains relatively constant but the nuclear charge increases, the effective nuclear charge experienced by the outer electrons increases across the period.
5. Therefore, the trend is that \(Z_{\text{eff}}\) increases from left to right across a period for main-group elements.