For a nuclide, ZA​XMass number =AAtomic number = Number of protons =ZNumber of neutrons, N=A−ZFor 92235​U,Mass number =235Number of protons =92Number of neutrons, N=A−Z=235−92=143The atom comprising a very small nucleus (centre) in which the protons (positively charged) and neutrons (neutral) are present. It is surrounded by electrons.