It seems you're asking about the nucleus of a Barium-138 atom (Ba-138). Here's the breakdown:

* Barium (Ba): This is the element, with an atomic number of 56. This means a neutral barium atom has 56 protons.

* 138: This is the mass number, indicating the total number of protons and neutrons in the nucleus.

Therefore, the nucleus of a Ba-138 atom contains:

* 56 protons

* 82 neutrons (138 total nucleons - 56 protons = 82 neutrons)

Let me know if you'd like to know more about the nuclear structure of other atoms!