Question

1. (a) The structure of fluorite (CaF2) maybe considered as a simple cubic arrangement of fluoride ions with the charge balancing Ca2+ ions being located in cubic holes.

   1. What is the coordination number of the calcium ions?

   2. What is the coordination number and geometry of the fluoride ions?

   3. What fraction of the cubic holes are filled by Ca2+ ions? (****please explain this one very clearly? and how the occupy of hole will differ if it was FCC instead?)

   4. How does this structure compare to that of CsCl?

2. (b) Describe and explain the differences between CO2 and SiO2 in terms of their bonding and physical properties.

3. (c) Draw an example of a cyclic silicate with Sin, (where 3 ≤ n ≤ 6), giving its formula and charge.

Thank you so muchh

Answer 1

Ca²⁺ centres are eight-coordinate, Each F⁻ centre is coordinated to four Ca²⁺ centres..The same crystal structure is found in numerous ionic compounds with formula AB₂, such as CeO₂, cubic ZrO₂, UO₂, ThO₂, and PuO₂.

• As there are two tetrahedral sites available for every calcium ion, the fluoride ions occupy all the tetrahedral sites. The stochiometry of the compound is 1:2.
• CsCl has a cubic structure that consists of an infinite chain of ions. The Cs (or Cl) ions sit at the eight corners of the cube and the Cl (or Cs) sit at the center of the cube
• SiO2 has a chain structure and Si is bonded to four oxygen atoms in the lattice structure.
  Due to the presence of vacant d-orbitals in Silicon, it can expand its covalency.
  After excitation of 3s electron into 3p orbital, four orbitals having each unpaired electron is formed. Each of the unpaired electron bonds with one 2p electron of Oxygen atom and hence a chain structure is formed.
  But CO2 is linear in structure. Carbon bonded to two oxygen atoms by double bonds.
  Ultimately, if the chemical structure of two compounds is different then their physical and chemical properties will also differ from each other.

• Cyclic silicates contain (SiO₃)_n^2n- ions which are formed by linking three or more tetrahedral SiO₄^4- units cyclically. Each unit shares two oxygen atoms with other units.

  

  E.g.     1) Benitoite - BaTi(SiO₃)₃ : contain three tetrahedra arranged cyclically.

             2) Beryl - Be₃Al₂(SiO₃)₆ : contain six-silicate rings. It is an aluminosilicate. Each aluminium is surrounded by 6 oxygen atoms octahedrally.