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Which among the following pairs of compounds is not isomorphous.
A. \[{\text{NaN}}{{\text{O}}_3}\]and \[{\text{CaC}}{{\text{O}}_3}\]
B. \[{K_{^2}}S{O_4}\]and \[{K_2}Se{O_4}\]
C. \[NaCl\,\]and\[KCl\]
D. \[NaF\]and \[MgO\]

Answer Verified Verified
Hint: Two or more substances have similar chemical formula, and polarizability.The ratio of the cation and anion radii are generally comparable and they exist in the same crystalline form or structure are isomorphous.

Complete answer:
Two or more solid substances having the same crystal structure are said to be isomorphous (iso-same, morphous-form) solids and the phenomenon is called isomorphism. \[NaN{O_3}\,\]and \[CaC{O_3}\]are isomorphous because their crystal structures are similar. Sodium nitrate and calcium carbonate are isosteric compounds and comparable size of cation and anion. \[{K_{^2}}S{O_4}\,\]and \[{K_2}Se{O_4}\] are isomorphous because of their crystal structures are similar and are isosteric compounds.
\[NaF\,\]and\[\,MgO\] are isomorphous because of their crystal structures are very similar in shape. Sodium fluoride and magnesium oxide are actually isosteric compounds, i.e they have the same number of valence electrons, but they differ in the type of atoms that form them. But \[NaCl\]and\[KCl\] have the same atomic ratio, similar molecular formula and similar chemical properties. But they have different crystal structures. Thus \[NaCl{\text{ }}and{\text{ }}KCl\] are not isomorphous.

Hence, the correct option is (C).

Note: Isomorphous substances are almost identical. The substances usually have similar chemical formula and Polarizability and ratio of cation and anion radius are generally comparable. The unit cell dimensions will be slightly different because of the different sizes of the atoms involved.
Existence of a substance in two or more crystalline forms having similar chemical composition but different arrangement of constituent particles is polymorphism. For example \[ZnS\] in the form of Zinc blende and wurtzite. In case of elements polymorphism is also called allotropy and all crystalline allotropes are polymorphs to each other.