Q. ) What is a primitive cell?
Ans. A primitive cell has particles (atoms) at all corners
of unit cell.
Q. ) Name the crystal system in which all the three axes are of
equal length which are inclined at the same angle but
the angal is not equal to 90º?
Q. )In a close packing of N spheres, how many
Ans. (i) Tetrahedral = 2N,
(i) tetaralhedral ,
and (ii) octahedral sites are present?
(ii) Octahedral = N.
Q. ) Arrenge simple cubic, body central cubic, face centred cubic and hexagonal
close packing in the increasing order of packing efficiency ?
Ans. Simple cubic < body centred cubic < face centred cubic = hexagonal close
Q. ) (a) What is meant by the term coordination number ?
Ans. Coordination number gives the number of nearest neighbours with which
a given sphere is in contact.
(b) What is the coordination number of atoms
(i) in a cubic close packed structure
(ii) in a body centred cubic structure ?
(b) (i) 12
Q. ) The ions of NaF and MgO all have the same number of electrons and the
internuclear distances are about the same (235pm and 215pm). Why then
are the melting point of NaF and MgO so different (992ºC and 2642ºC) ?
Ans. In MgO, both the ions carry two units of charge (Mg2+, O2-) where as in
NaF, each ion carries only one unit charge (Na+F-). Therefore, there are
stronger electrostatic froces of attraction in MgO and, hence,more energy
is required to overcome these forces. Thus ,its melting point is high..
Q .) Name the (i) most symmetrical and
Ans. (i) Most symmetrical crystal system : Cubic
(ii) most unsymmetrical crystal system>
(ii) Most unsymmetrical crystal system : Triclinic..
Q .) How many atoms can be assigned to unit cell if an element froms
Ans. (i) 2 (ii) 4.
(i) a body centred cubic cell and
(ii) face centred cubic cell ?
what is the maximum coordination number of an atom in a hcp crystal
structure of an element. ?