How many atoms are in Al2(SO4)3

here are two aluminum ions and three sulfate ions; or two aluminum, three sulfur, and twelve oxygen atoms.

answer : a model is often used to explain and make things easier to understand when any scientific topic needs to be demonstrated and explained.

as in this case where a scientist wants to use a model to present the results of his detailed scientific investigation. he should do it as it will him to make the audience understand the concept in a much better way. also, it can be changed according to new scientific understandings.