A molecule is the smallest unit of a substance that has all the properties of that substance. For instance, awater molecule is the smallest unit that is still water. A water molecule can be divided into tiny parts calledatoms. This produces two hydrogen atoms and one oxygen atom. But these atoms alone do not have the properties of water.

Some molecules make up thechemical elements. The chemical elements are the substances of which allmatter is made. That is, everything in the world is either a pure element or a combination of two or more elements. The molecules of pure elements contain only one type of atom. For example, the molecules of the element iron consist only of iron atoms.

A few elements have molecules that are made of a single atom. For instance, a molecule of helium gas consists of one helium atom. But most elements have molecules that contain at least two atoms joined together. For example, a molecule of oxygen gas consists of two oxygen atoms.

Other molecules contain atoms of two or more different elements. A substance made of such molecules is called achemical compound. Water is a chemical compound because its molecules have two hydrogen atoms and one oxygen atom. The sugar called glucose is another chemical compound. Each glucose molecule contains 6 carbon atoms, 12 hydrogen atoms, and 6 oxygen atoms. Some very complex molecules in living cells are made up of hundreds of thousands of atoms.

Strong chemical bonds hold the atoms in a molecule together. The bonds are formed when atoms share particles called electrons. Electrons are found either alone or in pairs in the outer part of atoms. When two atoms with unpaired electrons approach each other, the unpaired electrons may form a pair. Both atoms then share the pair. This holds the atoms together.