Looking at the periodic table how can you tell how many valence electrons an atom will have?

The Group number of a non-transition metal can be used to find the number of valence electrons in an atom of that element. The ones place of the group number is the number of valence electrons in an atom of these elements

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.