An isotope of Xenon has an atomic number of 54 and contains 77 neutrons. What is the mass number of this xenon isotope?

The mass number of an isotope is determined by the sum of the number of protons and neutrons in its nucleus. In this case, xenon has an atomic number of 54, which indicates that it has 54 protons. Given that the isotope contains 77 neutrons, you would combine the number of protons and neutrons to find the mass number.

By adding the 54 protons to the 77 neutrons, the calculation is straightforward:

54 (protons) + 77 (neutrons) = 131 (mass number).

Thus, the mass number of the xenon isotope is 131. This understanding is essential, as it reflects the composition of the nucleus and distinguishes between different isotopes of an element, which can have the same number of protons but different numbers of neutrons.