Final answer:
A compound is a substance composed of two or more elements joined by chemical bonds.
Step-by-step explanation:
A compound is a substance composed of two or more elements joined by chemical bonds. They have a unique composition that is always the same, and the smallest particle of a compound is called a molecule. For example, water is a compound that always contains one atom of oxygen and two atoms of hydrogen, with the chemical formula H2O. Compounds can have different properties than the elements that make them up.