Ionic bonding and lattice enthalpy
- What fraction of one edge centred octahedral void lies in one unit cell of fcc? Answer: 4. 1/4 Key idea: An octahedral void at an edge centre is shared by the 4 unit cells meeting at that edge. An fcc cell has one octahe... 1
- A compound is formed by two elements A and B. The element B forms cubic close packed structure and atoms of A occupy 1/3 of tetrahedral voids. If the formula of the compound is A_xB_y, then the value of x + y is in option Answer: 2. 5 Key idea: In ccp, if B atoms number N, there are N octahedral and 2N tetrahedral voids. Take 1 unit cell: B = 4 atoms; tetrahed... 1
- Given below are two statements : one is labelled as Assertion (A) and the other is labelled as Reason (R).Assertion (A) :In a particular point defect, an ionic solid is electrically neutral, even if few of its cations are missing from its unit cells.… Answer: 3. Both (A) and (R) are correct but (R) is not the correct explanation of (A) Key idea: missing cations with overall neutrality desc... 1
- Right option for the number of tetrahedral and octahedral voids in hexagonal primitive unit cell are : Answer: 4. 12, 6 Key idea: in a close-packed structure with n atoms per cell, there are n octahedral voids and 2n tetrahedral voids. A hexag... 1
- A compound is formed by cation C and anion A. The anions form hexagonal close packed (hcp) lattice and the cations occupy 75% of octahedral voids. The formula of the compound is : Answer: 3. C ₃ A ₄ In hcp, if there are N anions then there are N octahedral voids. Cations occupy 75% of octahedral voids, so cations = 0.7... 1
- Among CaH_2, BeH_2, BaH_2, the order of ionic character is Answer: 1. BeH ₂ < CaH ₂ < BaH ₂ Ionic character of a metal hydride increases with the electropositive (metallic) character of the metal, wh... 1
- Which is the incorrect statement ? Answer: 3. Frenkel defect is favoured in those ionic compounds in which sizes of cation and anions are almost equal. Key idea: Frenkel defec... 1
- In calcium fluoride, having the fluorite structure, the coordination numbers for calcium ion (Ca^2+) and fluoride ion (F^-) are Answer: 2. 8 and 4 Key idea: in the fluorite structure, Ca²⁺ forms an fcc lattice and F^- occupies all tetrahedral holes. Each F^- is surrou... 1
- The ionic radii of A^+ and B^- ions are 0.98×10^-10 m and 1.81×10^-10 m. The coordination number of each ion in AB is : Answer: 1. 6 Key idea: the radius ratio decides the coordination number. r_+/r_-=0.98/1.81=0.54 This lies in 0.414-0.732, which means octahe... 1