Here's why:

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

* Valence Shell: The valence shell is the outermost shell of an atom, which in this case is the third shell (n=3).

* Valence Electrons: The electrons in the valence shell are called valence electrons and are involved in chemical bonding.

Therefore, sodium has one valence electron in its 3s orbital.