Work Step by Step
When naming an isotope, the element (in this case platinum) is listed and this name is followed by a number, which in this case is 196. This number is the atomic mass number, and it equals the number of protons plus the number of neutrons. Because all platinum atoms have 78 protons (this is the atomic number of platinum, which can be found on the periodic table), the atomic mass number of the platinum isotope with 118 neutrons is 196 (78 protons + 118 neutrons = 196). Thus, the isotope is called platinum-196.