Coordination number is also called ligancy of central atom in a molecule. It can be equal to the number of bonds formed by the ligand(s) with the central atom. ∵ In C2O42−, it bears (−2) charge and form total 6 bonds with the central atom (Cr). ∴ Coordination number =Six‌(6) Oxidation state is also called oxidation number. The oxidation number of K=+1 The oxidation number of C2O42−=−2 Let, oxidation number of Cr=x Then, in K3[Cr(C2O4)3] value of ′x′ is calculated as follows. 3×+1+x+3×−2=0 ∴x=+3 Hence, oxidation state of Cr in K3[Cr(C2O4)3] is +3.