First established by the French chemist Joseph Proust in 1797 this law states that if a compound is broken down into its constituent chemical elements, then the masses of the constituents will always have the same proportions by weight, regardless of the quantity or source of the original substance. The second was the law of definite proportions. The first was the law of conservation of mass, closely associated with the work of Antoine Lavoisier, which states that the total mass in a chemical reaction remains constant (that is, the reactants have the same mass as the products). Near the end of the 18th century, two laws about chemical reactions emerged without referring to the notion of an atomic theory. About a century later it was discovered that Dalton's atoms are not actually indivisible, but the term stuck.ĭalton's law of multiple proportions From A New System of Chemical Philosophy, 1808. In the early 19th century, the scientist John Dalton noticed that chemical elements seemed to combine with each other by discrete units of weight, and he decided to use the word 'atom' to refer to these units, as he thought these were the fundamental units of matter. Modern atomic theory is not based on these old concepts. This ancient idea was based in philosophical reasoning rather than scientific reasoning. The word atom is derived from the ancient Greek word atomos, which means 'uncuttable'. The basic idea that matter is made up of tiny indivisible particles is an old idea that appeared in many ancient cultures.

Particles which are truly indivisible are now referred to as 'elementary particles'. John Dalton applied the term to the basic units of mass of the chemical elements under the mistaken belief that chemical atoms are the fundamental particles in nature it was another century before scientists realized that Dalton's so-called atoms have an underlying structure of their own. The term 'atom' comes from the Greek word atomos, which means 'uncuttable'. By the end of the 19th century, atomic theory had gained widespread acceptance in the scientific community. The concept that matter is composed of discrete particles is an ancient idea, but gained scientific credence in the 18th and 19th centuries when scientists found it could explain the behaviors of gases and how chemical elements reacted with each other.

The current theoretical model of the atom involves a dense nucleus surrounded by a probabilistic 'cloud' of electronsĪtomic theory is the scientific theory that matter is composed of particles called atoms. For the modern view of the atom which developed from atomic theory, see atomic physics. For a history of the study of how atoms combine to form molecules, see history of molecular theory. This article is about the historical development of understanding the existence and behavior of atoms.

Atomic molecular theory