Question

Alkaline earth metal oxide not having rock metal structure is _______
A.$BeO$
B.$MgO$
C.$CaO$.
D.$SrO$

Answer 1

Hint: We all know rock salt is nothing but $NaCl$. So salts similar to its structure, properties and behavior will be having the same structure. Find out one such quality in any one of the options given in the structure.

Complete step by step answer:
We all know that Sodium Chloride ($NaCl$) is a salt, formed from the association of Sodium ($N{a^ + }$) ions and Chloride ($C{l^ - }$) ions. The characteristic rock salt structure is the actual structure of Sodium Chloride, which looks like a solid strong structure. So, the answer must be a salt. All the options given in the questions are salts. So, let us start characterizing them with another behavior. Sodium Chloride ($NaCl$) is an ionic compound or salt. Let us check whether all the salts given are ionic compounds. Magnesium Oxide $\,(MgO)\,$ is a salt from the association of Magnesium $\,M{g^{2 + }}\,$ ions and Oxide (${O^{2 - }}$) ions. Hence it can form a rock salt structure and it is not the right answer. Calcium Oxide ($CaO$) is formed by the ionic attraction between Calcium ($C{a^{2 + }}$) ions and Oxide (${O^{2 - }}$) ions. Hence it is an ionic compound too and it also forms a rock salt structure.
And, coming to $BeO$, it is not an ionic compound. It is a covalent compound formed from the mutual electrons sharing of Beryllium ($Be$) and Oxygen ($O$). It is a salt but not an ionic compound. Only ionic salts form rock salt structure and since Beryllium Oxide is not an ionic salt, it does not form rock salt structure.
Hence the answer for the given problem is Beryllium Oxide ($BeO$).

Note:
$\,BeO\,$has hexagonal wurtzite form at normal temperature. This differs with other Group $\,2\,$ members whose oxides, $\,MgO,{\text{ }}CaO,{\text{ }}SrO,{\text{ }}BaO\,$ have a framework of cubic rock salt. The arrangement changes to a tetragonal shape at high temperatures. Beryllium oxide is a nonionic special metallic oxide compound.