Final answer:
The mass number of the tin isotope with 50 protons and 66 neutrons is 116.
Step-by-step explanation:
The mass number of an isotope is the sum of its protons and neutrons. For the isotope of tin with 50 protons and 66 neutrons, the mass number (A) is calculated as 50 (protons) + 66 (neutrons), equaling 116. Therefore, the mass number of this tin isotope is 116. The mass number uniquely identifies a specific isotope of an element and is crucial in understanding its atomic structure.