Its atom contains 50 electrons and 70 neutrons

Tin-120 because an atom has to have a neutral charge. If an atom has 50 electrons it must have 50 protons unless it's an ion. Add 70 to the atomic mass because it has 70 neutrons on top of 50 protons. That equals to 120. Tin has an atomic number of 50 (meaning it has 50 protons) and it has roughly an atomic mass of about 118. If the given has 70 neutrons then it's a isotope of tin.