Molecule
Molecule, smallest unit of a substance that shows all the chemical properties of that substance. A molecule is a group of atoms that are bound tightly together by strong chemical bonds called covalent bonds. Every molecule has a definite size. If a molecule is broken up into its atoms or into smaller groups of atoms by chemical processes, these pieces will not behave like the original molecule. A molecule can contain atoms of the same element or atoms of different elements. A substance made up of molecules that include two or more different chemical elements is called a molecular compound. An example of a molecular compound is water. Water is made of molecules that contain two hydrogen atoms and one oxygen atom. See also Atom . Many substances on Earth are made of molecules. Millions of molecules join together to make up the cells in humans or in any other plant or animal. The food we eat, the air we breathe, the clothes we wear, and the wood, paint, and carpeting that we use in home