Which statement is true about effective nuclear charge (Z_{eff})?

• A. Z_{eff} is always equal to the atomic number (Z) of an element.
• B. Z_{eff} decreases as you move down a group.
• C. Z_{eff} remains constant for all elements in the same period.
• D. Z_{eff} increases across a period from left to right.

Answer: (D)

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 (\(Z\)) minus the shielding effect caused by other electrons.
2. Recall the formula for effective nuclear charge: \[Z_{eff} = Z - S\] where \(Z\) is the atomic number (number of protons) and \(S\) is the shielding constant representing electron repulsion.
3. Analyze the trends in the periodic table: as you move across a period from left to right, the atomic number \(Z\) increases by one for each successive element, but the shielding \(S\) does not increase as much because electrons are added to the same principal energy level.
4. Because shielding does not increase significantly across a period, the increase in \(Z\) leads to an increase in \(Z_{eff}\), meaning electrons feel a stronger pull from the nucleus.
5. Conversely, moving down a group adds new electron shells, increasing shielding significantly and thus decreasing \(Z_{eff}\) experienced by outer electrons, so \(Z_{eff}\) does not remain constant within a period or equal to \(Z\).