I WILL MARK YOU BRAINLIST PLEASE HELP

 See below

Explanation:  a.  The mass of an element is composed of:

protons:     1 amu each

neutrons:   1 amu each

electrons:   0 amu each

Only the protons and neutrons are counted in the atomic mass of an element

b.  Electrons are assigned a mass of 0.  They do have a mass, but it is exceedingly small compared to the protons and neutrons, so they are left out of the calculation of an element's mass.

c.  An element becomes unstable if the neutrons exceed the protons by a certain ratio, dependent on the element.

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.

b. examine and describe cells