How did J. J. Thomson's discovery that atoms contain electrons change Dalton's atomic theory?
Well, Dalton's atomic theory states that atoms are indivisible (meaning atoms cannot be broken any further into smaller particles) but because J.J Thomson proved that electrons exist, J.J suggested that atoms are divisible. This changed Dalton's theory because Dalton's theory suggested no subatomic particles (protones, nuetrons, ELECTRONS) are present but through the cathode-ray tube experiment, Thomson found that atoms can be broken down further into further subatomic particles (pure electrons), which contradicted Dalton's theory.
Sir Joseph John Thomson, along with his team, discovered that electrons were present in atoms and that electrons were actually particles and not waves, atoms, molecules, et cetera. This disproved one important point of Dalton's atomic theory:
Atoms cannot be subdivided, created, or destroyed.
Although atoms cannot be created or destroyed, it was now proven that atoms can be subdivided. The Plum Pudding model of the atom by Sir Thomson clearly differentiated between the positive and negative regions of an atom, which was against Dalton's theory. As a result, Dalton's theory was changed due to the discovery of the subatomic particle: electron.