Which of the following ground-state electronic configurations represents an element that exhibits a — Periodic Table and Periodicity Chemistry Question

💡 Solution & Explanation

Elements with fully filled or exactly half-filled stable configurations resist the addition of an extra electron. Configuration C belongs to Magnesium ($Mg$), which has a completely filled $3s^2$ subshell. Adding an electron forces it into the higher energy $3p$ orbital, requiring an input of energy (positive $\Delta_{eg}H$).