Electronic config of Na (Z=11).

**The reasoning** Sodium (Na) has 11 electrons that need to be arranged in shells around the nucleus. Remember the **2-8-8 rule** (also called the octet rule): - First shell (K) holds maximum **2 electrons** - Second shell (L) holds maximum **8 electrons** - Third shell (M) holds maximum **8 electrons** So we fill them in order: - K shell: 2 electrons (9 remaining) - L shell: 8 electrons (1 remaining) - M shell: 1 electron (0 remaining) **Electronic configuration = 2,8,1** **Why the wrong options tempt you** **B) 2,8,3** — You might miscount or confuse sodium with aluminum (Al, Z=13), which is actually 2,8,3. **C) 2,1,8** — This breaks the filling rule! You can't leave the second shell half-empty and jump to fill the third shell. Shells fill sequentially from innermost to outermost. **D) 1,8,2** — This violates the first shell rule. The K shell MUST be filled first with its maximum of 2 electrons before moving outward. **Quick takeaway** Fill electron shells like filling seats on a bus: **complete the front rows (inner shells) before moving to back rows (outer shells)** — always 2, then 8, then 8.