Here's why:

* Electron Configuration: Sulfur's electron configuration is 1s² 2s² 2p⁶ 3s² 3p⁴.

* 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 called valence electrons. In sulfur's case, the 3rd shell contains 6 electrons (2 in the 3s orbital and 4 in the 3p orbitals).