Here's why:

* Electron Configuration: Sodium has an atomic number of 11, meaning it has 11 electrons. Its electron configuration is 1s²2s²2p⁶3s¹.

* Shell Structure: The first shell (n=1) can hold a maximum of 2 electrons. The second shell (n=2) can hold a maximum of 8 electrons. The third shell (n=3) can hold a maximum of 18 electrons.

* Sodium's Electrons: Sodium fills its first and second shells completely (2 + 8 = 10 electrons). The remaining electron goes into the third shell.

Therefore, sodium has only three electron shells, with the third shell being partially filled.