Question

Which of the following oxides have rock salt structure with coordination number 6:6?
A.BeO
B.MgO
C.CaO
D.SrO

Answer 1

We know that rock crystals are ionic compounds with chemical formula NaCl and it occurs naturally in the white crystalline forms. Its structure is formed by repeating fcc (face centred cubic) unit cells.

Complete step by step answer:
Let’s discuss all the options one by one to determine the coordination number of the compounds.

Option A is BeO. The compound BeO is formed by hexagonal close pack arrangement of unit cells. Its structure is Wurtzite type possessing coordination number 4:4. In the structure of BeO, oxide ions $\left( {{{\text{O}}^{2 - }}} \right)$in hcp and beryllium ions $\left( {{\text{B}}{{\text{e}}^{{\text{2 + }}}}} \right)$ occupy tetrahedral voids. So, the coordination number of BeO is not 6:6. Therefore, A is not the correct answer.
Option B is MgO. MgO is the structure formed by repeating fcc unit cells. Its structure is similar to NaCl possessing coordination number of 6:6. So, MgO is a rock salt structure.
Option C is CaO. In CaO also, the structure is formed by repeating fcc unit cells. So, its coordination number is also 6:6. Hence, it is also a rock salt structure.
Option D is SrO. In SrO also, the structure is formed by repeating fcc unit cells. So, its coordination number is also 6:6. Hence, it is also a rock salt structure.

So, the correct answer is Option B,C,D.

Note: The smallest repeating unit in a crystal is termed as unit cell, body centred cubic cell, primitive cubic unit cell and face centred unit cell. A face centred cubic unit cell has atoms at centre of all faces and at all corners of the cube.