A compound is a substance formed when two or more chemical elements are chemically bonded together. In mixtures, the substances present are not chemically bonded together.
The elements in any compound are always present in fixed ratios.
Example 1: Pure water is a compound made from two elements - hydrogen and oxygen. The ratio of hydrogen to oxygen in water is always 2:1. Each molecule of water contains two hydrogen atoms bonded to a single oxygen atom.
Example 2: Pure table salt is a compound made from two elements - sodium and chlorine. The ratio of sodium ions to chloride ions in sodium chloride is always 1:1.
Example 3: Pure methane is a compound made from two elements - carbon and hydrogen. The ratio of hydrogen to carbon in methane is always 4:1.
Example 4: Pure glucose is a compound made from three elements - carbon, hydrogen, and oxygen. The ratio of hydrogen to carbon and oxygen in glucose is always 2:1:1.
Compounds can be decomposed chemically into their constituent elements.
Further differences between compounds and mixtures are listed in the definition of mixture.