The answer is gallium-72. The isotope of an atom containing 31 protons and 39 neutrons suddenly has 2 neutrons added to it. So, it has 31 protons and 41 neutrons (39 + 2 = 41). The mass number of an element is the sum of the number of its protons and neutrons: 31 + 41 = 72. The atomic number of an element is the number of its protons. This isotope has 31 protons and its atomic mass is 31. According to the Periodic table, gallium has atomic mass of 31. Thus the isotope is gallium-72.
