Given the unidentified isotope 137 67X, which number stands for protons, electrons, and neutrons?

From this unidentified isotope, the value of 137 would represent the sum of protons and neutrons present. While the value below it, if written in nuclear notation represents the number of protons, in this case 67. To determine the number of neutrons, you simply need to subtract the mass number by the atomic number, the number of protons.

Assuming that this is an example of a neutral isotope that is unidentified the number of electrons would be equal to the number of protons present. So there would be 67 electrons.