（1）But if you could sort out the atoms in all that matter, you would find that there are only about 100 kinds. Each of those 100 kinds of atoms is a different size.

（2）Some kinds of matter are made up of only one kind of atom. These kinds of matter are called elements.

（3）Iron is an element, too. It is made of just iron atoms.

（4）But most kinds of matter are made of two or more different kinds of atoms joined together. These kinds of matter are called compounds.

（5）Water is a compound made of two elements, an element called oxygen and an element called hydrogen. By themselves, oxygen and hydrogen are invisible gases.