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theories and models on atoms

Discoverortheory and model
Democritus (400 B.C.) *Called nature's basic particle atom - Greek for invisible *Believed that atoms were tiny indivisible particles
John Dalton (1808) *British Chemist *All matter is composed of extremly small particles called atoms *Atoms of a given element are the same in size, mass and other properties; atoms *Atoms cannot be subdivided, created, nor destroyed *Imagined atoms as tiny, solid balls.
J.J. Thomson (1897) *British Scientist *Discovered the electron *Stated that atoms are electrically nuetral, they must contain a positive charge to balance the negative charge. *Described the atom as a positively-charged sphere with electrons enbedded in it.
J.J. Thomson (1897) *Because electrons have so much less mass than atoms, atoms must contain other particles that account for most of their mass. *His model is described as the 'plum pudding' model. The pudding represents the protons, and the plums represent the electrons.
Hanataro Nagaoka (1904) *Japanese physicist. *Suggested that the atom is constructed in the same way as our solar system.
Hanataro Nagaoka (1904) *Proposed a model of the atom that had a large sphere in the center with a positve charge and showed the electrons revolving around this sphere, like the planets around the sun.
Ernest Rutherford (1911) *British physicist. *Conclused that the atom is mostly empty space. *Discovered positive core center of an atom. *The volume of the nucleus is very small compared to the total volume of an atom.
Ernest Rutherford (1911) * His model showed electrons located outside the positive nucleus and orbited randomly.
Neils Bohr (1913) *Danish physicist. *HIs model showed that electrons are not randomly located around the nucleus and can circle the nucleus only in specific paths, or shells. *Stated that atoms absorb or give off energy when the electrons move from one shell to the other.
James Chadwick (1932) *British physicist. *Discovered the nuetron. *Stated that the nuetron has about the same mass as the proton, but no electrical charge. *The existence of the nuetron explained why atoms are heavier than the total mass of their protons and electrons.
Modern Model (1932-Present) *This model of the atom came from the work of many scientists, over many decades. *The model shows the electrons forming a negatively charged cloud around the nucleus. *It is impossible to determine exactly where an electron is at any given time.
Created by: tatischreiber