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The ratio of cations to anion in a closed pack tetrahedral is:
(b) r+/ r- for tetrahedral void=0.225-0.414;
r+/ r- for triangular=0.155-0.225
A spinel is an important class of oxides consisting of two types of metal ions with the oxide ions arranged in ccp layers. The normal spinel has 1/8th of the tetrahedral void occupied by one type of metal and one half of the octahedral voids occupied by another type of metal ions. Such a spinel is formed by Zn2+, Al3+ and O2- with Zn2+ in tetrahedral void. Give the simplest formula of the spinel.
(A) Let the no. of tetrahedral voids = a
No. of octahedral voids = a/2
1/8 th tetrahedral voids is occupied by Zn2+
No. of Zn2+ =a/8
1/2 of octahedral voids occupied by Al3+
No. of Al3+ = a/4
Ratio of Zn2+: Al3+
1 : 2
For 1 mole Zn2+, Al3+=2
Total +ve charge = 2+3x2=8 therefore, No. of O2- =4
The simplest formula will be ZnAl2O4
Close packing is maximum in the crystal lattice of:
(b) The maximum packing or the maximum proportion of volume filled by hard spheres in various arrangements are:
(a) simple cubic=π/6=0.52
(b) bcc=π/√3/8=0.68
(c) fcc=π/√2/6=0.74
(d) hep=π/√2/6=0.74
(e) Diamond=π/√3/6=0.34
Frenkel defect is noticed in:
(d) Frenkel defect is arised when the cations are missing from their lattice sites and occupy interstitial sites. As a result of Frenkel defect, density remains unchanged but dielectric constant increases.
How many octahedral and tetrahedral holes are present per unit cell in a face centred cubic arrangement of atoms?
(C) In fcc octahedral voids: at the centre = 1
at the edges = 12x1/4=3
Total =4
TiO2 is well known example of:
(b) TiO2 has tetragonal system with five plane of symmetry and five axes of symmetry.
Which pairs shows isomorphism?
(c)
NaNO3 and KNO3 are not isomorphs because they have same molecular formula but different crystal structure.
Two or more substances having same crystal structure are said to be isomorphous. They contain constituent atoms of the substance in the same atomic ratio.
, and are isomorphous but and are not isomorphous as they have different crystal structures even-though they have same atomic ratio, similar molecular formula and similar chemical properties.
The elements of symmetry in a crystal are:-
(d) A crystal has these three types of symmetry.
Oxygen atoms forms fcc unit cell with 'A' atoms occupying all tetrahedral voids and 'B' atoms occupying all octahedral voids. If atoms are removed from two of the body diagonals then determine the formula of resultant compound formed.
originally, Number of oxygen atoms = 1/8x8+1/2x6 = 1+3 = 4 atoms
Number of 'A' atoms = 8
Numbe of 'B' atoms = 1/4x12+1=3+1=4atoms
After removal of atoms from two body diagonals are-
Number of oxygen atoms = 4-1/8x4 = 4-1/2 =7/2
Number of 'A' atoms = 8-4 = 4
Numbe of 'B' atoms = 4-1 = 3
Hence, the formula of the compound is
A4B3O7/2 or A8B6O7
A solid has a structure in which W atoms are located at the carners of a cubic lattice. Oxygen atom at the centre of the edges and Na atom at centre of the cube. The formula of the compound is
Number of 'W' atoms = 8*1/8=1(Since there are at the corners)
Number of 'O' atoms = 12*1/4 = 3(become 'O' atoms are at the centre of face edges)
Number of 'Na' atoms = 1x1=1(The Na atom is at the centre of cube)
then the formula will be NaWO3
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