Iron sulfide is a chemical compound (different from a mixture of Iron and Sulfur, show by its different physical and chemical properties). It is made of iron and sulfide ions. It has iron in its +2 oxidation state. It is found in the ground as the mineral pyrite. It is shiny like a metal, but it does not conduct electricity. It can make sparks when it is powdered (ground finely). It does not always contains sulfide ions; sometimes it contains a mixture of sulfide and sulfur. When it is exposed to oxygen in air, it forms acids which pollute streams and rivers. It reacts with acids such as hydrochloric acid to make hydrogen sulfide gas. In a mixture of Iron and Sulfur, one could separate the Iron from the mixture using a magnet. The Iron Sulfide compound (commonly made by melting the mixture over high heat), however, cannot be separated by a magnet, marking the compound and the mixture as distinct from each other. Where the mixture has the distinct properties of both Iron and Sulfur, the compound has combined properties that are different (i.e., reduced magnetic attraction, different melting point, etc.)
Iron is an element, its symbol in the periodic table is Fe and its melting point is 1538°C. Sulfur is also an element, its symbol in the periodic table is S and its melting point is 115.2°C. Iron is a metal and its atomic mass is 56. Sulfur is a non-metal and its atomic mass is 32. Iron is a silvery grey color, Sulfur is a yellow color. After a chemical reaction (heat), the two elements, Iron and Sulfur, are chemically joined together to form Iron Sulfide, which is a compound. The symbol for this compound is FeS, and the solid Iron Sulfide is a metallic black color.