An atom of sodium (Na, Z=11) has electron configuration:

## The reasoning Sodium has 11 electrons (because Z = 11 means 11 protons, and atoms are neutral, so electrons = protons). We fill electron shells using the **2n² rule**: - **First shell** (n=1): Maximum 2 electrons → **2** - **Second shell** (n=2): Maximum 8 electrons → **8** - **Third shell** (n=3): Remaining electrons → **1** (since 2 + 8 + 1 = 11) So the configuration is **2,8,1**. This makes sodium highly reactive because that single electron in the outermost shell is easily lost to form Na⁺ ions (which is why sodium is in Group 1 of the periodic table!). ## Why the wrong options tempt you **B) 2,8,3** – You might confuse sodium with aluminum (Al, Z=13), which actually has 2,8,3. **C) 2,1,8** – This breaks the filling rule! You must completely fill inner shells before moving outward. The second shell can hold 8, so you can't leave it with just 1. **D) 1,8,2** – Same mistake—you can't put 8 electrons in the second shell before filling the first shell's 2 spots. ## Quick takeaway **Fill shells in order: 2 → 8 → 8 → ... and never skip ahead!** Count electrons = atomic number, then distribute properly.