Here's why:

* Valence electrons are the electrons in the outermost shell of an atom. They are the ones involved in chemical bonding.

* Sulfur's electron configuration: Sulfur (S) has an atomic number of 16, meaning it has 16 protons and 16 electrons. Its electron configuration is 1s² 2s² 2p⁶ 3s² 3p⁴.

* Outermost shell: The outermost shell for sulfur is the 3rd shell, which contains 6 electrons (2 in the 3s subshell and 4 in the 3p subshell).

Important note: The mass number (33 in this case) doesn't affect the number of valence electrons. The mass number tells us the total number of protons and neutrons in the nucleus, but the number of valence electrons is determined by the element's electron configuration.