Microbiology: A Human Perspective 7th Edition by Eugene Nester,Martha Nester,Denise Anderson, Evans Roberts

Exercise 1

The basic unit of all matter, the atom, is composed of protons, electrons, and neutrons. The four most important elements in biology are carbon, hydrogen, oxygen, and nitrogen. In diagrams, atoms can be shown as a cloud structure, symbol designation, or Lewis structure.
Why are electrons not considered in determining the mass number of an element?