Step-by-step explanation:
One of the Dalton's postulates states that:
The atoms of same element are similar in the shape and the mass and other properties but different from atoms of other elements.
But the existence of isotopes violates this postulate because isotopes are the species which have same atomic number but different atomic mass. For example, carbon exist in three isotopes which are C-12, C-13 and C-14 and have different mass and some properties and is a contradiction of this postulate.