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Robert Boyle - 1661 proposed that chemical elements could not be made simplier

John Dalton - 1803

Atoms were indivisible spheres

Elements were formed of atoms

Atoms of different elements had different masses

Henri Becquerel discovered radioactivity which chowed that atoms could be split

1896

JJ Thomson discovered the electron and found out it was negatively charged

Electrons were the same in all elements

Electrons were light so there must be something else accunting for the rest of the mass of the atom

He came up with the Plum PUDDING MODEL