Sulfur, periodic table S, is a solid with 16 protons. It's most common isotope is S32 with 16 neutrons. Other stable isotopes exist in small amounts.
Sulfur is the tenth most common element by mass in the universe, and the fifth most common on Earth. Sulfur is common on the Martian surface. The Martian mantle is probably richer in sulfur, chlorine and phosphorus than Earth's.