In order:
electrons
electrons
protons
4
3
3
The amount electrons and protons balance out their charges, leading the charge to be 0
This answer depends on what you learned in class:
They discovered the presence of neutrons after experimenting.
or
They realized that these neutrons had to exist to make a stable nuclei and balance out the mass.
(both answers are correct, so you can combine them if you want)
