Body centered cubic structure (CsCl structure). The coordination number of the unit cell is eight.

What is the coordination number of Cs+ and Cl ions in the CsCl structure?

This is obvious if we compare the CsCl unit cell with the simple cubic unit cell showing the interstitial site. We may also consider the neighborhood in which each ion finds itself. Each Cs⁺ is surrounded by 8 Cl^- (so the Cs⁺ coordination number is 8) at the corners of each cube.

What is the coordination number of Cl (-) in CsCl structure?

The coordination number of Cl− in CsCl structure is 8.