#Z#, the atomic number, is simply the number of protons, fundamental, massive, positively charged particles, found within the nucleus. This number defines the element. Of course, the nucleus could contain different numbers of neutrons, massive, neutrally charged particles, and this gives rise to the existence of isotopes. Here the isotope is #""^235U#, so there are #235-92 =143# neutrons. This is the fissile isotope, approx. 0.72% naturally abundant.