Scientist use models to study atoms because you cannot either see or perceive directly what do atoms look like.
Models permit to explain in a simplified way how the atoms are constituted, and, so, figure out how they behave and how they are the bulding blocks of the matter (elements and compounds).
A model can be a drawing or structure with balls and sticks, which show in an idealized way what an atom is.
Since the scientists have discovered different facts about the atoms along the last 200 years the models of the atoms have also evolved.
The plum pudding model of the atom proposed by JJ Thompson was different of the nuclear model proposed by Rutherford, and Rutherford's was different from the planetary model proposed by Bohr, and this is different from the quantum model.
