Chemical compounds are substances composed of two or more elements united chemically in definite proportions by mass. An example of a compound is water. Water is naturally a liquid composed of hydrogen, a gas and oxygen, a gas. Together they form liquid with different characteristics. This union between different elements to form different substances is the essences of chemical compounds. Basically, this is the reason for studying them separate - compounds and elements both have their own characteristics that a student must identify.

You may want to use the example of water to further demonstrate the different of properties.

compounds
molecules
elements