Here's why:

* Electron Configuration: Sodium has an electron configuration of 1s² 2s² 2p⁶ 3s¹.

* Outermost Shell: The outermost shell is the 3rd shell (n=3).

* Valence Electron: The 3s¹ orbital contains the single valence electron.

This single valence electron is what makes sodium highly reactive and likely to lose it to form a positive ion (Na⁺).