Here's why:

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

* Outer Shell: The outermost shell is the third shell (n=3).

* Outermost Electrons: The 3s and 3p sublevels contain a total of 6 electrons (2 in 3s and 4 in 3p).

Therefore, sulfur has 6 electrons in its outer shell, which makes it a reactive element.