Multiple choice

Match the entries in Group – I with molecules in Group – II and select the correct option ||| |---|---| |Group – I |Group – II| |A. Rock-salt structure|1. MgO| |B. Fluorite structure|2. Na2S| |C. Anti-fluorite structure|3. CaF2| |D. Sphalerite structure|4. ZnS|

  1. A - 1, B - 2, C - 3, D - 4

  2. A - 1, B - 3, C - 2, D - 4

  3. A - 2, B - 1, C - 4, D - 3

  4. A - 3, B - 2, C - 4, D - 1

  5. A - 3, B - 2, C - 1, D - 4
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Rock-salt structure: Anions occupy the lattice points of expanded face centred cubic lattice and the cations occupy all the octahedral holes. The limiting radius ratio is in between 0.414 and 0.732. The co-ordination numbers of cation and anion are (6:6). The general formula of ionic compounds having this structure is AB. Examples: NaCl, KCl, MgO, CaO etc. Fluorite structure: The cations are arranged into cubic close packing and the anions occupy all the tetrahedral voids. Thus, there are four cations and eight anions per a unit cell. The formula of ionic compound having this structure is A4B8 or AB2. The co-ordination numbers of cation and anion are (8:4). Examples: CaF2, ZrO2, ThO2, BaF2, BaCl2, SrCl2, PbCl2 etc. Anti-fluorite structure: Anions are arranged in cubic closest packing and cations occupy all the tetrahedral voids. There are four anions and eight cations per unit cell of this structure and hence, the general formula of an ionic compound is A8B4 or A2B. The ideal radius ratio is between 0.225 - 0.414 (but this ratio is not always maintained). The co-ordination numbers of cation and anion are (4:8). Examples: Na2O, K2O, Li2O, Rb2O, K2S, Cl2O, Na2S etc. Zinc-blende or sphalerite structure: Anions occupy the face centred cubic lattice points and cations occupy half of the tetrahedral holes. The ideal radius ratio is in between 0.225 to 0.414. There are four anions and four cations in the unit cell. Therefore, the formula is A4B4 or AB. The co-ordination numbers of cation and anion are (4:4). Examples: ZnS, BeO etc.

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