Why was John Dalton's explanation of gas atoms in mixtures later found to be wrong?

John Dalton’s explanation for why gases in a mixture behave independently was based on the erroneous assumption that only like atoms repel one another, while unlike atoms react indifferently to each other.1

While this premise was scientifically incorrect, it served a functional purpose in his work by helping to explain why each gas in a mixture exerts its own partial pressure, a concept now known as Dalton’s law. By proposing this, Dalton successfully demonstrated that atoms of different elements are distinct entities, which was a significant step in the development of modern atomic theory.213

Additionally, Dalton made a faulty assumption regarding atomic combinations, believing that molecules of an element were always single atoms. For example, he incorrectly hypothesized that water was formed by one atom of hydrogen and one of oxygen (HO) rather than the correct H₂O. These early misconceptions were eventually corrected as the scientific understanding of molecules and kinetic theory evolved.34