The coordination number of ${\mathrm{Cl}}^{-}$ ion in CsCl crystal is

CsCl has body-centred cubic (bcc) arrangement. Each Cs⁺ ion is surrounded by 8 Cl⁻ ion and each Cl⁻ is surrounded by 8 Cs⁺ ions.

Hence, The coordination number of ${\mathrm{Cl}}^{-}$ ion in CsCl crystal is 8