Here's why:

* Electron Configuration: Sodium's electron configuration is 1s² 2s² 2p⁶ 3s¹.

* Valence Shell: The outermost shell (the 3rd shell in this case) is called the valence shell.

* Valence Electrons: The electrons in the valence shell are the valence electrons. Sodium has only one electron in its valence shell.