The best replacement for isotope is ion.

The best alternatives to isotope are rated as following:
★★★★★  ion
★★★  isomer
★★  masonry
★  allotrope
★  nuclide
★  radioactive isotope
★  radioisotope
★  packery
  quicksand
  isotone
  isotope
  isobar

Your alternative for isotope is not listed? Suggest!
Speaking about isotope, one probably shouldn't forget about 13 protons and 14 neutrons.

Other search terms related to isotope and their ratings are:
★★★★★ 13 protons and 14 neutrons
★ 4 protons and 5 neutrons
★ infinite scroll
★ lazy load
15 protons and 16 neutrons
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