How many protons are there in one atom of hydrogen

There is only one proton because it has a molecular mass of one

i believe you are right for all except the 3rd ( look at sqdancefan his answer is much better)

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.