An atomic model shows 19 protons, 20 neutrons, 19 electrons. What is the mass number of the atom?
The mass number of an atom is equal to the sum of the number of protons and neutrons. In this case, the atom has 19 protons and 20 neutrons, so the mass number would be:
19 protons + 20 neutrons = 39
Therefore, the mass number of the atom is 39.