# Xenon has an atomic number of 54. A particular isotope of xenon has a mass number of 131. How many protons and how many neutrons does each atom of that isotope have?

""^131Xe has $77$ neutrons.......
Now we have elemental xenon, and thus we KNOW that the nucleus contains $\text{54 protons}$. If it didn't possess 54 protons it would not be elemental xenon, and would be some other element. And thus the mass number is the sum of the massive nuclear particles: $54 + 77 = 131$, and hence the ""^131Xe isotope. This site tells me that ""^131Xe has 21% isotopic abundance.