Sulfides are compounds derived from the sulfide anion, S2-. As sulfur is a member of group 16 on the periodic table, it is considered a chalcogen, and sulfide compounds belong to a class of compounds known as chalcogenides.
The bonding in transition metal sulfides is highly covalent, a property that is intimately related to their ability to function as semiconductors and pigments . These are the sulfides most often used directly as functional materials; they are found as pigments, catalysts, optical materials, phase change materials, solid electrolytes, or semiconductors. One particularly familiar sulfide is the bright yellow species CdS or "cadmium yellow". This is the color used for school buses in the United States, though the color is now replicated with alternate pigments due to the toxicity of cadmium.
Another familiar transition-metal sulfide is the black tarnish formed on sterling silver: silver sulfide. Cadmium sulfide is an essential component of cadmium telluride (CdTe) solar cells. Calcium polysulfide ("lime sulfur") is a traditional fungicide in gardening. Lead sulfide is used in infra-red sensors. Molybdenum disulfide, found naturally as the mineral molybdenite, is used as a petrochemical catalyst to remove sulfur from fossil fuels and as a solid lubricant for high temperature and high pressure applications. Zinc sulfide is used for lenses and other optical devices in the infrared part of the spectrum. Zinc sulfide with a trace of copper is used for photoluminescent strips for emergency lighting and luminous watch dials. Research into properties of semiconducting sulfides in the form of nanostructured materials such as quantum dots has grown in recent years, and such materials are now coming into use in optoelectronic applications.
Alkali metal and alkaline earth chalcogenides, including the sulfides, have bonds with more ionic character than those found in transition metal sulfides. They are typically colorless, water-soluble compounds, and are used more as chemical reagents than as functional materials. For instance, sodium sulfide is an important industrial chemical, used in paper manufacturing, dyes, leather tanning, crude petroleum processing, treatment of heavy metal pollution, and others.