Matter & Energy

Matter is composed of atoms or groups of atoms called molecules. The arrangement of particles in a material depends on the physical state of the substance. In a solid, particles form a compact structure that resists flow. Particles in a liquid have more energy than those in a solid. They can flow past one another, but they remain close. Particles in a gas have the most energy. They move rapidly and are separated from one another by relatively large distances.


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 homes are all made of molecules. Millions of different molecules exist in nature or can be made by chemists. The nature of each molecule depends on the atoms that it contains and how they link to each other. For example, the oxygen that animals require is made of molecules that have two oxygen atoms bound together. If one oxygen atom binds to a carbon atom, the molecule is instead the poisonous gas carbon monoxide.

Scientists study molecules and their structures so they can better understand why substances behave the way they do. For example, molecular structure helps explain why water boils at a high temperature. Scientists and manufacturers also use their knowledge of molecules and molecular structures to make substances with desirable properties. Plastics, for instance, are laboratory-made substances that consist of enormous molecules containing thousands of atoms. By manipulating the molecular structure of plastics, chemists have created materials that stretch better, resist fading, or can be used in microwave ovens without melting. Similarly, pharmaceutical chemists use their knowledge of molecular structure to develop new drugs that more effectively ease pain or fight disease. The discovery of the structure of deoxyribonucleic acid (DNA), the molecule that contains the genetic blueprint for living organisms, opened the door to tremendous advances in medicine and industry. Knowledge of the structure of DNA has enabled physicians to understand and treat certain genetic diseases. Moreover, by manipulating DNA structure, scientists have been able to modify—or genetically engineer—organisms, creating, for example, bacteria that produce valuable drugs (see Genetic Engineering).