​Objectives

• ​Explain what a compound is.

• ​Give examples of compounds

Compound​

A substance that is formed when atoms of two or more elements join together.​

​All the substances in the figure are different from the elements you learned about in Lesson 2. The substances in the figure are each made of two or more different kinds of atoms. When two or more atoms of different elements join together, the substance that forms is called a compound.  A compound has properties that are different from the properties of the elements that form the compound.

​Think again about a molecule of water. The drawing shows that an atom of oxygen combines with two atoms of hydrogen to form a molecule of the compound water. Water is different from the elements that form it. Water is a liquid. Both oxygen and hydrogen are gases. You will learn more about breaking down the compound water into its elements when you do the investigation. 

​Another compound that probably is familiar to you is table salt. The chemical name of salt is sodium chloride. It is formed when the element sodium is combined with the element chlorine. Sodium chloride is very different from each of the elements it contains. Sodium is solid. you might be surprised to learn that chlorine is a poisonous gas. However, when chlorine is combined with sodium to form sodium chloride, chlorine no longer has its poisonous property. Remember that a compound can have completely different properties from the elements that form it. 

​Most kinds of matter on Earth are compounds. In fact, there are more than 10 million known compounds. The table list some common compounds and tells the elements that make up each compound.

​You might wonder if you can tell by looking at a substance whether it is an element or a compound. An unknown substance must be tested in a laboratory to determine whether it is an element or a compound.