What is the difference between an atom's mass number and its atomic number

atomic mass is associated with the number of neutrons and protons are present in a particular nucleus of element. atomic number is usually a number of protons present in an element's nucleus, it's the average weight of an element and total number of nucleons in the atomic nucleus

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.