Question

The 8:8 type of packing is present in:
a.) $Mg{F_2}$
b.) CsCl
c.) KCl
d.) None

Answer 1

Hint: The 8:8 type of packing is shown by the molecule in which both cation and anion are equal in number. Further, both the anions have coordination numbers of 8. It forms a cubic unit cell.

Complete step by step answer:
This question is from solid state. And here the packing means the packing of the ions forming the different types of structures. We will start by knowing something about 8:8 type of packing. The 8:8 type of packing means that the one cation will be coordinating with eight anions and one anion will be coordinating with the eight cations. Overall 1:1 ratio of cations and anions will be present in the molecule.

Now, let us understand the coordination number.
Coordination number is defined as the number of atoms of ions that are present just after this atom we are talking about or one can also say that it is the number of atoms or ions with which the atom is in just direct contact with. The coordination number of an ion decides its packing.
Now, as we are done with the conceptual portion. Let us see the options.
The options a.) has $Mg{F_2}$. In this molecule, the Mg has coordination number 6 and F has 3. So, this does not suit. It will be from 6:3 packing. Thus, option a.) is incorrect.
Now, the option b.) having CsCl. The CsCl molecule has Cs with coordination number 8 and Cl with coordination number 8. Thus, it will form 8:8 packing.
So, option b.) is the correct answer.
The next is the option c.) KCl molecule. The K has coordination number 6 while Cl has 12. So, it will be 6:12 lattice. Thus, option c.) is also incorrect.
So, the correct answer is “Option B”.

Note: In Cesium chloride lattice, the cesium atoms are present at all the corners of the cube forming cubic lattice while all the chloride ions are at the interstitial sites. This type of structure is taken up by many other molecules. In such a situation to adopt CsCl type structure, the cations and anions must be of similar sizes like the Cs and Cl ions.