Chemistry is the study of atoms, molecules, and the interactions they undergo, called chemical reactions. Through the breaking and forming of atomic bonds, various compounds change into new compounds, either producing or consuming energy in the process. Energy may be released in the form of heat or light, as in a fire or explosion. By exploiting what scientists know about chemical reactions, chemicals with useful properties can be produced.
The word derives from the slang term for the older term, "alchemist." Modern chemistry is generally thought to have begun in the 17th century, most prominently with the experiments and writings of Robert Boyle. It was Boyle that formulated Boyle's law, which holds that the temperature and volume of a gas are related.
Another important milestone was the theory of conservation of mass, developed by Antoine Lavoisier in 1783. The law holds that no mass is created or destroyed in a chemical reaction — if someone burns a log in an enclosed container, the container will weigh exactly as much as it did prior to the reaction, only it will have changed to ash. Because of his efforts in popularizing chemistry and making it more accessible to the common person, Lavoisier is often regarded as the father of modern chemistry.
Another huge milestone in the development of the field was the formulation of the atomic theory by John Dalton around 1800. Atomic theory holds that substances are made of small individual units called atoms that link together through atomic bonds. An aggregation of bonded atoms is called a molecule. Atoms and molecules make up almost everything that people can see, and interact intimately with energy, often in the form of electromagnetic radiation. It was soon realized that electromagnetic radiation can be released when the electrons in an atom transition to a lower energy state.
Today, chemistry is a fundamental part of civilization. It is used to manufacture just about everything, and chemical processing is ubiquitous and consumes a major portion of humanity's total energy budget. Being a chemical engineer is one of the highest-paying scientific careers.