an atomic model shows 19 protons, 20 neutrons, and 19 electrons. What is the mass number of the atom?
a. 58.
b. 39.
c. 20.
d. 19.
The mass number is equal to the sum of the protons and neutrons in an atom. Since there are 19 protons and 20 neutrons, the mass number would be 19 + 20 = 39.
Therefore, the correct answer is b. 39.