Question

Coordination number in ABAB type arrangement is:
A.$6$
B.$8$
C.$16$
D.$4$

Answer 1

Here, we need to find out the coordination number in ABAB type arrangement. Coordination number of an atom is basically, the total number of atoms or molecules or ions which are bonded to the central atom.

Complete answer:
Now first let’s see what kind of arrangement ABAB is.
In the ABAB type of arrangement, each one of the spheres is connected to the other six spheres present in the structure. This kind of arrangement is known as hexagonal close packing.
In the hexagonal close packing, as the name suggests it is in the hexagonal shape. Now, here in this, each one of the spheres is connected with the other present sphere. And they are arranged in this manner:
The second layer of these particles, they lie below the gaps of the first layer of the particles and the third layer of these sphere shaped particles, they lie below the gaps of the second layer of these particles and these particles are arranged in such a manner that each one of the sphere shaped particles are connected with the other six sphere shaped particles and the center of these lie at the corners of the regular hexagonal, therefore, it is known as hexagonal close packing.
This type of arrangement is known as the ABAB type of arrangement. Also, in such a type of arrangement the smallest $3D$ $3D$ building unit is known as an HCP unit cell.
Hence, we can conclude that in ABAB type of rearrangement, the coordination number is $6$.
Therefore, the correct option is- Option $A$- $6$.

Note:
It can be noted that the coordination number of the building units or unit cells, which in the case of hexagonal close packing is known as HCP, is the characteristic property of the arrangement or the structure which never changes in any conditions.