Final answer:
A compound is a chemical substance that forms when two or more different elements combine in a fixed ratio. Water (H₂O) and sodium chloride (NaCl) are examples of compounds. Compounds have a chemical formula that describes the ratio of atoms of each element.
Step-by-step explanation:
A compound is a chemical substance that forms when two or more different elements combine in a fixed ratio. Water (H₂O), for example, is a compound that is made up of two hydrogen atoms for every one oxygen atom. Sodium chloride (NaCl) is a compound made up of one sodium atom for every chlorine atom. An important characteristic of a compound is that it has a chemical formula, which describes the ratio in which the atoms of each element in the compound occur.