Question 10
The mass number of an atom is the sum of which particles?
The right answer is the one marked above.
Protons + neutrons. Written as A, it counts the particles gathered in the nucleus.
The electrons are left out on purpose. An electron has about 1/1840 of the mass of a proton — set beside the protons and neutrons it is small enough to ignore entirely, so the mass of an atom is decided by the nucleus alone.