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(A) In an $NaCl$ structure,$Cl^{-}$ ions form a $CCP$ (or $FCC$) lattice. The total number of $Cl^{-}$ ions per unit cell is $4$.An axis passing throu

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$2$

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(A) In an $NaCl$ structure,$Cl^{-}$ ions form a $CCP$ (or $FCC$) lattice. The total number of $Cl^{-}$ ions per unit cell is $4$.An axis passing through two opposite face-centers passes through two face-centered ions and the body-center (which is an octahedral void occupied by $Na^{+}$).The ions touching this axis are the two face-centered $Cl^{-}$ ions.When these two face-centered $Cl^{-}$ ions are removed,the contribution of the remaining face-centered $Cl^{-}$ ions is calculated as follows:Original contribution of face-centered ions = $6 	imes \frac{1}{4} = 1.5$.After removing $2$ face-centered ions,the remaining face-centered ions are $4$.New contribution of face-centered ions = $4 	imes \frac{1}{4} = 1$.The contribution of corner $Cl^{-}$ ions remains $8 	imes \frac{1}{8} = 1$.Total effective number of $Cl^{-}$ ions = $1 + 1 = 2$.

If all ions touching an axis passing through two opposite face-centres of unit cell are removed in $NaCl$ (rock salt) structure in which $Cl^{-}$ ions form $CCP$ and $Na^{+}$ ions occupy all octahedral voids,then the effective number of $Cl^{-}$ ions per unit cell is:

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