Molecules

Molecules are groups of two or more atoms held together by chemical bonds. They are among the basic building blocks of many substances found in nature and in everyday life. A molecule may contain atoms of the same element, such as oxygen gas made of two oxygen atoms, or atoms of different elements, such as water, which contains two hydrogen atoms bonded to one oxygen atom.

The arrangement of atoms within a molecule determines its shape and strongly influences its physical and chemical properties. Molecules vary greatly in size. Some are made of only a few atoms, while others, including many biological molecules such as proteins and DNA, contain thousands or even millions of atoms linked together in complex structures.

Molecules are constantly moving, and their motion depends on temperature and the state of matter. In gases they move freely, in liquids they remain close together while flowing past one another, and in solids they mainly vibrate around fixed positions. During a chemical reaction, existing bonds may break and new ones form, creating different molecules without changing the atoms themselves.

Scientists study molecules to understand how matter behaves and how living organisms function. This knowledge has led to advances in medicine, materials science, agriculture and environmental protection. Many familiar products, including medicines, plastics, soaps, perfumes and food flavorings, are designed by understanding and controlling the properties of molecules.