Question

$ CsBr $ crystallises in a body centred cubic unit lattice with an edge length of $ 4.287\mathring{A} $ . How many molecules of $ CsBr $ will be present in the unit lattice ?

• A. 1
• B. 2
• C. 3
• D. 4

Answer 1

Answer: (A)

In $CsBr$ unit cell structure $Br^-$ ions are arranged in a simple cubic arrangement. In this arrangement the $Br^-$ ions are present at all the comers of the cube. The $Cs^+$ ions occupy the cubic sites i.e., $Cs^+$ ions are present at the body centre of the cube. Number of $Br^-$ ions $= 8 \times \frac{1}{8} = 1$ ions per unit cell.
There is also one $Cs^+$ ion in the body centre and its contribution per unit cell is $1$.
Therefore, there is one $Br^-$ ion for each $Cs^+$ ion. There is one molecule of $CsBr$ in the unit lattice.