Final answer:
John Dalton was correct regarding the existence of atoms in his atomic theory, but incorrect about their indivisibility and uniform composition within the same element due to the later discovery of sub-atomic particles and isotopes.
Step-by-step explanation:
John Dalton, often considered the father of modern atomic theory, was right about the existence of atoms, but he was wrong about their indivisibility. Dalton's atomic theory posited that all matter is composed of atoms and that these atoms are indivisible. However, later discoveries proved that atoms can indeed be subdivided into smaller particles such as protons, neutrons, and electrons.
Dalton's theory also stated that atoms of the same element are identical in all respects. This part of the theory was modified when the concept of isotopes was discovered, which showed that atoms of the same element can have different masses. Despite these inaccuracies, Dalton's work laid the foundation for modern chemistry, although aspects related to the composition of atoms and their sub-atomic particles were not correctly understood in his model.