To get Polyatomic Ion Particles in The First Descendant, you can kill any mission monster on Fortress. Mission monsters are the powerful yellow bar enemies that can spawn in during missions.

If you're got two ions, the one with more oxygens is 'ate' and the one with less is 'ite'. I don't think there's ever any cases where you have more than two different ions--and the difference is ...

The camo challenges for Polyatomic only become available once you've unlocked the platinum camo for 51 weapons - the original number of guns available at launch in Modern Warfare 2.