21. Given the four complexes ( A ) [Co(H₂O)₆]³⁺, ( B ) [Co(NH₃)₆]³⁺, ( C ) [Co(NH₃)₆]²⁺ and ( D ) [Co(NH₃)₄]²⁺, which order of ligand field splitting (Δ) of Co-complexes is correct? (A) A ＞ B＞ D ＞ C (B) B ＞ A ＞ C＞ D (C) A ＞ B ＞ C ＞ D (D) C ＞ A ＞ B ＞ D (E) None of the above is correct.

4. Both ZnS and CaF₂ have a face-centered cubic unit cell where the S²⁻ and Ca²⁺ ions are closest packed in each structure. Which of the following is true? (A) There are 4 tetrahedral holes empty in each structure. (B) In both compounds, one-half of the tetrahedral holes are filled. (C) In both compounds, all the tetrahedral holes are filled. (D) There are 8 Zn²⁺ ions and 4 F⁻ ions in the unit cell. (E) In ZnS, one-half of the tetrahedral holes are filled by Zn²⁺ ions, whereas in CaF₂ all the tetrahedral holes are filled with F⁻ ions.

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