* Fixed Ratio: Iron sulfide (FeS) always has a specific, fixed ratio of iron (Fe) to sulfur (S) atoms. This ratio is determined by the chemical formula.

* New Properties: When iron and sulfur combine to form iron sulfide, they create a new substance with properties distinct from either iron or sulfur alone. For example, iron sulfide has a different color, melting point, and chemical reactivity than either of its constituent elements.

* Chemical Bonding: The iron and sulfur atoms in iron sulfide are held together by chemical bonds, meaning they are chemically combined rather than just physically mixed.

In contrast, a mixture would have:

* Variable Composition: The ratio of components in a mixture can vary.

* Retention of Properties: The components in a mixture retain their original properties.

* No Chemical Bonding: The components in a mixture are simply mixed together physically and not chemically bonded.