Final answer:
A compound is a substance made up of two or more different elements that are joined together in a fixed ratio. Compounds have a chemical formula that describes the ratio of atoms of each element in the compound.
Step-by-step explanation:
A compound is a substance made up of two or more different elements that are joined together in a fixed ratio. For example, sodium chloride (NaCl), iron sulphide (FeS), calcium carbonate (CaCO₃), and water (H₂O) are all compounds.
Compounds have a chemical formula that describes the ratio of atoms of each element in the compound. Naming compounds and writing their chemical formula involves knowing the elements in the compound, the number of atoms of each element in the compound, and their positions in the periodic table.