As you study in class 9th the delton theory in chemistry there are some drawbacks. You study in ninth only three drawbacks and other you will study in further classes.
The three drawbacks are as follow:-
(i) One of the major drawbacks of dalton’s atomic theory of matter is that atoms were thought to be indivisible (which cannot be divided). We now know that under special circumstances , atom can be further divided into still smaller particles called electrons ,protons and neutrons. So, atoms are themselves made up of three particles : electrons , protons and neutrons.
(ii) Dalton’s atomic theory says that all the atoms of an element have exactly the same mass. It is , however, now known that atoms of same element can having slightly different masses .
(ii) Dalton’s atomic theory said that atoms of different element having different masses. It is , however, now know that even atoms of different elements can have the same masses