the iodine isotope with 72 neutrons Enter the isotopic symbol in the form X-A . For example, C-13. he phosphorus isotope with 14 neutrons Enter the isotopic symbol in the form X-A . For example, C-13. the uranium isotope with 145 neutrons Enter the isotopic symbol in the form X-A . For example, C-13. the argon isotope with 22 neutrons Enter the isotopic symbol in the form X-A . For example, C-13.