Here's why:

* Atomic Number: Sodium has an atomic number of 11, meaning it has 11 protons and 11 electrons.

* Electron Configuration: The electron configuration of sodium is 1s² 2s² 2p⁶ 3s¹. This means:

* The first shell (n=1) has 2 electrons (1s²)

* The second shell (n=2) has 8 electrons (2s² 2p⁶)

* The third shell (n=3) has 1 electron (3s¹)

Therefore, sodium has three electron shells.