Dalton found that 'carburetted hydrogen gas' contains twice as much hydrogen per measure of carbon as 'olefiant gas', and concluded that a molecule of 'olefiant gas' is one carbon atom and one hydrogen atom, and a molecule of 'carburetted hydrogen gas' is one carbon atom and two hydrogen atoms. According to Thomson, Dalton's idea first occurred to him when experimenting with 'olefiant gas' ( ethylene) and 'carburetted hydrogen gas' ( methane). In 1804, Dalton explained his atomic theory to his friend and fellow chemist Thomas Thomson, who published an explanation of Dalton's theory in his book A System of Chemistry in 1807. This suggested that each element combines with other elements in multiples of a basic quantity. He noticed a pattern that later came to be known as the law of multiple proportions: in compounds which contain two particular elements, the amount of Element A per measure of Element B will differ across these compounds by ratios of small whole numbers. John Dalton studied data gathered by himself and by other scientists. Dalton's law of multiple proportions From A New System of Chemical Philosophy (John Dalton 1808). Finally, there was the law of definite proportions, established by the French chemist Joseph Proust in 1797, which states that if a compound is broken down into its constituent chemical elements, then the masses of those constituents will always have the same proportions by weight, regardless of the quantity or source of the original compound. Lavoisier also defined the law of conservation of mass, which states that in a chemical reaction, matter does not appear nor disappear into thin air the total mass remains the same even if the substances involved were transformed. Lavoisier showed that water can be decomposed into hydrogen and oxygen, which in turn he could not decompose into anything simpler, thereby proving these are elements. This brought an end to the ancient idea of the elements of matter being fire, earth, air, and water, which had no experimental support. The first was Antoine Lavoisier redefining an element as a substance which scientists could not decompose into simpler substances by experimentation. Near the end of the 18th century, a number of important developments in chemistry emerged without referring to the notion of an atomic theory. In the early 19th century, the scientist John Dalton noticed that chemical substances seemed to combine with each other by discrete and consistent units of weight, and he decided to use the word atom to refer to these units. 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. Then physicists discovered that these particles had an internal structure of their own and therefore perhaps did not deserve to be called 'atoms', but renaming atoms would have been impractical by that point.Ītomic theory is one of the most important scientific developments in history. Then the definition was refined to being the basic particles of the chemical elements, when chemists observed that elements seemed to combine with each other in ratios of small whole numbers.

Initially, it referred to a hypothetical concept of there being some fundamental particle of matter, too small to be seen by the naked eye, that could not be divided. The definition of the word 'atom' has changed over the years in response to scientific discoveries. 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.

Bohrs model of an atom