Answer is: Protons and neutrons are located in the nucleus.
Atomic number (Z) is total number of protons and mass number (A) is total number of protons and neutrons in a nucleus.
Nucleus of an atom has positive charge, electrons are negative and atom has neutral net charge.
Helium (symbol: He) is an element with atomic number 2 (2 protons) and mass number 4 (2 neutrons; n° = 4 - 2 = 2).
