Dalton's theory was based on the premise that the atoms of different elements could be distinguished by differences in their weights. His theory proposed a number of basic ideas:
* All matter is composed of atom.
* Atoms are indivisible and indestructible.
* Different elements have different types of atoms.
* All atoms of the same element are identical.
* Chemical reactions occur when atoms are rearranged.
* Compounds are formed by a combination of two or more different kinds of atoms.
Using his theory, Dalton rationalized the various laws of chemical combination which were in existence at that time. However, he made an error in assuming that the simplest compound of two elements must be binary, formed from atoms of each element in a 1:1 ratio, and his system of atomic weights was not very accurate for he gave oxygen an atomic weight of seven instead of eight. Despite the errors, Dalton's theory provided a logical explanation of concepts, and led the way into new fields of experimentation.
J. J. Thomson
In 1904 Thomson suggested a model of the atom as a sphere of positive matter in which electrons are positioned by electrostatic forces. This discovery dramatically changed the modern view of the atom. Thomson's work suggested that the atom was not an "indivisible" particle as John Dalton had suggested but, a jigsaw puzzle made of smaller pieces.
After the discovery of the electron and proton as subatomic particles J.J. Thomson had started to discover atomic theory that gave a complete explanation of atomic structure. According to Thomson protons are embedded in the atoms like a water melon and electrons are present in atoms to make the atom electrically neutral. He called this the “plum pudding” model, in the model the electrons and protons are uniformly mixed throughout the atom. His discovery of atomic theory however, was not able to explain the atomic structure properly.
Ernest Rutherford published... [continues]
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