Question

How did the discovery of isotopes help to lead to a revised model of atomic structure?

Answer 1

Dalton postulated that atoms of the same elements had the same mass while atoms of different elements had different masses. 

The discovery of isotopes revealed that atoms of the same element did not necessarily have same masses, they could have different masses.

Therefore, the modern periodic table that we use nowadays contains the average atomic mass of all the isotopes found for a given element.