### Related Formula textExchange Energy (E) propto K cdot fracn(n-1)2$\text{Exchange Energy } (E) \propto K \cdot \frac{n(n-1)}{2}$ where n$n$ represents the total count of parallel spin electrons within degenerate electronic levels. ### Core Logic The enhanced quantum and physical stability belonging to half-filled and completely filled subshells is driven by explicit mechanisms: 1. **Symmetrical distribution**: Electrons in a half-filled subshell are distributed uniformly across all degenerate spatial orientations, minimizing structural polarization. 2. **Large Exchange Energy**: Parallel spins maximize the permissible quantum exchanges, dropping potential energy configurations significantly. 3. **Smaller Coulombic Repulsion**: Placing single electrons inside separate degenerate orbitals reduces electron-electron proximity repulsions compared to forced pairing. 4. **Smaller Shielding Effects**: Uniform spatial shells provide less mutual shielding, allowing closer nuclear interactions. ### Step 1: Assessing Validity Options Evaluating statement metrics: - Statements (A), (B), (D), and (E) outline accurate chemical factors. [cite: 1008, 1009, 1010, 1011] - Statement (C) incorrectly claims non-degenerate orbitals; half-filled configurations strictly populate *degenerate* subshell levels (like p_x, p_y, p_z$p_x, p_y, p_z$). ### Pattern Recognition Stability markers checklist: Symmetry, exchange energy maximization, and decreased coulombic repulsion are the dominant triad explaining half-filled orbital stability (e.g., textCr: 3d^5 4s^1$\text{Cr}: 3d^5 4s^1$ or textCu: 3d^10 4s^1$\text{Cu}: 3d^{10} 4s^1$ anomalies). ### Evaluation Rubric / Model Answer null ### Chapter Mix Class 11 Chemistry: Structure of Atom