Answer : The theory of atom was controversial in early 1900's because people still thought that things can continue to be further divided and sub-divided.
This was the opinions of the people during early 1900's which created controversies from the theory proposed. Back in minds of people it was hard for them to believe that atoms were the smallest part present in any matter. When it was proved with several other supporting theories in late 1900's people started to believe the theory of atoms.
