All atoms of the same element are exactly alikeand have the same mass. Atoms of differentelements are different and have different masses. An atom of one element cannot be changed into an atom of a different element. Even though he never made an experiment to prove his point, this is what he thought.
Let's go to 1830 and meet Michael Faraday
Faraday's breakthrough came when he wrapped two insulated coils of wire around an iron ring and found that, upon passing a current through one coil, a momentary current was induced in the other coil.
In 1830, Michael Faraday realized that John Dalton's predictions were wrong. That atoms do have electrons. Michael Faraday realized that John was wrong because he realized when he touched a doorknob he got shocked.
This phenomenon is now known as mutual induction. Following his experiments, he found that if he moved a magnet through a loop of wire an electric current flowed in that wire. That meant electrons were added to the atom.
Let's go to 1897 and meet JJ Thomson
On April 30, 1897, J.J. Thomson's experiments with cathode ray tubes showed that all atoms contain tiny negatively charged subatomic particles or electrons. Thomson proposed the plum pudding model of the atom.
This meant that he had positively charged matter with small negatively charged electrons scattered randomly throughout. He thought that electrons were embedded randomly in the positive part of the atom.
Let's go to 1911 and meet Ernest Rutherford
In 1911, Rutherford used a very thin piece of gold foil with small positively charged particles. Expecting most of the particles to be deflected slightly by the gold atoms (as predicted by Thomson's revision to the atomic theory) he was shocked to discover that most of the particles pass straight through empty space.