What element has 4 valence electrons and 3 orbital shells

A need is something that is necessary for an organism to live a healthy life. Needs are distinguished from wants.

Take it.... (^_-)

a sequence is a function whose domain is the set of natural numbers.

natural numbers are all the negative and positive integers

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.