The History Of The Atom Part 3

John Dalton and J.J. Thomson

By Aslam Sookia 13B

John Dalton


John Dalton (6 September 1766 – 27 July 1844) was an English chemist, meteorologist and physicist.
He is best known for his work on the development of the modern atomic theory.
His theory was that different atoms could be distinguished by their different weights and his theories came up with some ideas:

All matter is composed of atoms

Atoms cannot be made or destroyed

All atoms of the same element are identical

Different elements have different types of atoms

Chemical reactions occur when atoms are rearranged

Compounds are formed from atoms of the constituent elements.
One mistake he made was that he assumed that compounds had a 1:1 ratio of atoms of each element.
Dalton’s atomic weight system was not very accurate either; however, his new ideas brought new methods for experiments.


J.J. Thomson


Sir Joseph John “J. J.” Thomson, (18 December 1856 – 30 August 1940) was a British physicist. In 1897, Thomson showed that cathode rays were composed of a previously unknown negatively charged particle, which then lead on to the discovery of the electron.

Several scientists, such as William Prout and Norman Lockyer, had suggested that atoms were built up from a more fundamental unit, but they though that this unit would be the size of the smallest atom, hydrogen. Thomson, in 1897, was the first to suggest that the fundamental unit was over 1000 times smaller than an atom, suggesting the subatomic particles now known as electrons. He discovered this through his experiments with the cathode rays.

Thomson’s model of atomic structure is often called the plum pudding model. He suggested in 1904 that the negatively charged electrons that he had recently discovered were scattered throughout a cloud of positive charge, like the plums in plum pudding.



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