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In my textbook its mentioned that BeCO3 (beryllium carbonate) is an ionic compound, but going according to fajans rules, small cation with high charge density and large anion should give a majorly covalent compound.

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    $\begingroup$ Relatively small, electronegative oxygen atoms in large anions are less polarizable than big single-atom anions like iodide. $\endgroup$
    – Poutnik
    Mar 12 at 6:39
  • $\begingroup$ Beryllium carbonate is unstable. Mismatch is size is too large, and carbonate have a general tendency to break down to CO2 and oxides. $\endgroup$
    – Mithoron
    Mar 12 at 13:31
  • $\begingroup$ For delocalized electrons, it does provide some insight. However, strictly speaking, Fajan rule is only applicable to atoms and ions. Hard-Soft Acid-Base theory is more widely applicable for polyatomic ions. $\endgroup$
    – ananta
    Mar 12 at 18:58

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