Which of the following elements would you expect to be most reactive, based on the number of valence electrons it has?

C. Lithium

Explanation:

This image describes the trend in reactivity. Group 1 metals are always the most reactive so yeah :)

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.