Here's why:

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

* Full shells: The first two electron shells (1s and 2s) are completely filled with 2 electrons each. The 2p subshell is also filled with 6 electrons. This means the first two electron shells (1s, 2s, and 2p) are completely full.

* Partial shell: The 3p subshell has 4 electrons. It needs 2 more electrons to be fully filled.

Therefore, Sulfur has 6 full electrons in its 1s, 2s, and 2p shells.