Which One Is Most Ionic?

(i) Non metallic oxides are more covalent (or less ionic) as compared to metallic oxides.

(ii) Higher the polarising power of cation (higher for higher oxidation state of similar size cation’s) more will be covalent character.  

(iii) P₂O₅ will be more covalent than other metallic oxides. 

(iv) Oxidation state of Mn is +7 in Mn₂O₇, oxidation state of Cr in CrO₃ is +6 and oxidation state of Mn is 2 in MnO.  

  MnO is most ionic.  

P₂O₅, being a non-metallic oxide will definitely be more covalent than the other metallic oxides. Further, we know that higher the polarising power of the cation (highter oxidation state of the similar size cations) more will be the covalent character. Hence Mn is in +7 O.S in Mn₂O₇, Cr is +6 in CrO₃and Mn is +2 in MnO. So MnO is the most ionic and Mn₂O₇ is the most covalent.