Coordination Number of Rb in RbBr Unit Cell:

The coordination number of an ion in a crystal lattice refers to the number of oppositely charged ions surrounding it. In the case of RbBr, Rb^+ cation is surrounded by Br^- anions and vice versa.

Calculation:

To calculate the coordination number of Rb in RbBr, we need to consider the ionic radii of Rb and Br ions and the crystal structure of RbBr.

Crystal Structure of RbBr:

RbBr has a face-centered cubic (FCC) crystal structure. In this structure, each Rb^+ ion is surrounded by 6 Br^- ions and each Br^- ion is surrounded by 6 Rb^+ ions.

Calculation of Coordination Number:

To calculate the coordination number of Rb in RbBr, we need to consider the distance between Rb^+ and Br^- ions. As per the FCC structure, the distance between the centers of two adjacent ions is given by:

d = 2^(1/2) * r

where d is the distance between the centers of two adjacent ions and r is the ionic radius of the ion.

For RbBr, the distance between the centers of Rb^+ and Br^- ions is given by:

d(Rb-Br) = 2^(1/2) * (r(Rb) + r(Br))

d(Rb-Br) = 2^(1/2) * (148 pm + 195 pm) = 343.5 pm

Therefore, the coordination number of Rb in RbBr is given by the number of Br^- ions that can be accommodated around an Rb^+ ion within a distance of 343.5 pm.

Conclusion:

In the FCC structure of RbBr, each Rb^+ ion is surrounded by 6 Br^- ions within a distance of 343.5 pm. Hence, the coordination number of Rb in RbBr is 6.

Co- ordination no. is 8 structural arrangement is cubic