The atomic number of Xenon is 54, so there are 54 subatomic particles inside the nucleus of a Xenon atom.
The mass number of Xenon is 131. Since the atomic number is 54, the difference between the mass number and the atomic number gives the number of neutrons in the nucleus. Therefore, there are 131 - 54 = 77 subatomic particles outside the nucleus in a Xenon atom.