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.

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