Bohr model of either Na or Cl, describe it and tell what features it has (3 rings or shells, with one of the numbers showing the # of , etc)

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.