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.

Answer 1

The mass number of an atom is the sum of the number of protons and neutrons. In this case, the atom has 19 protons and 20 neutrons:

Mass number = number of protons + number of neutrons
Mass number = 19 + 20 = 39

So, the mass number of the atom is 39. The correct answer is B. 39.