What are the failure of Dalton's atomic theory?

An atom cannot be further divided into still smaller particles. Atom can neither be created nor be destroyed in the chemical process. The atom of one element are different from those of any other elements. Atoms of different elements make combine in simple whole number ratio to form compound atoms of a new substance.

Why was Dalton's theory rejected?

According to Dalton atoms of different elements combine in simple whole number ratio to form compounds. This is not seen in complex organic compounds like sugar C12H22O11. The theory fails to explain the existence of allotropes.