unsur 138The following table shows the atomic nuclei that are isotonic (same neutron number N = 81) and isobaric (same nucleon number A = 138) with Lanthanum-138. Naturally occurring isotopesName of the isotope: Cesium-138; Cs-138 Symbol: 138 Cs or 138 55 Cs Mass number A: 138 (= number of nucleons) Atomic number Z: 55 (= number of protons) Neutrons N: 83 Isotopic