How many protons, electrons, and neutrons does an atom with atomic number 50 and mass number 125 contain? a. 50 protons, 50 electrons, 75 neutrons. b. 120 neutrons, 50 protons, 75 electrons. c. 75 electrons, 50 protons, 50 neutrons. d. 70 neutrons, 75 protons, 50 electrons.

a. 50 protons, 50 electrons, 75 neutrons.

For a neutral atom, the atomic number is equal to number of protons and electrons. Therefore, this atom has 50 protons and 50 electrons. The mass number is the sum of the number of protons and neutrons. By difference, the number of the neutrons is 75. Thus, the answer is letter A. 

The correct answer is b