(b) The crystal lattice
(i) packing of the atoms/molecules in the crystal.
The crystal lattice is the repeat of atoms/molecules in the crsytal therefore
depends on how they pack to produce the lattice.
1 Mark
(c) The origin in reciprocal space
(iii) neither. This is a natural origin
that corresponds to the X-ray beam.
1 Mark
(d) The origin in the crystal
(ii) chosen by the crystallographer. The
origin is chosen so that there is high symmetry in the unit cell of the crystal.
1 Mark
(e) The space group symmetry of the crystal
(i) packing of the
atoms/molecules in the crystal. The space group is the classification of the
symmetry of the crystal which depends upon how the atoms/molecules are packed in
the crystal to give the particular symmetry operators.
1 Mark
Comment: Excellent
2. A Laue class is one of the 11 centrosymmetric point groups which describe
3-dimensional diffraction symmetry. Each crystal system has a corresponding
point group but the higher symmetry crystal systems, e.g. cubic, belong to 2
Laue groups. The Laue class for the triclinic system is -1 and the monoclinic
system belongs to the 2/m class.
Comment: Basically correct, but a bit too brief
4 Marks
3. Rotary inversions cannot be found in an enantiomorphic molecule because
there will be a centre of inversion in the molecule hence a centrosymmetric
molecule which cannot be chiral. Enantiomorphic molecules do not contain
mirror/glide planes.
Comment: The question is more concerned about the
symmetry between molecules, than the symmetry of enantiomorphic molecules
per se. You need to mention handedness explicitly.
2 Marks
4.Other coordinates:
(0.271, 0.25, 0.958) (0.729, -0.25, 0.542) (0.729,
0.75, 0.042)
Comment: One of your atoms is not inside the
unit cell
2 Marks
No the Co atom is not sited on a special position in this space
group.
Comment: Correct, but no reason given
1 Marks
5. I'm a bit confused, in the question the molecule is called
1,2-dichloroethylene but the diagram of the molecule is of 1,2-dichloroethane, I
will assume the question is about 1,2-dichloroethane.
Comment: Thanks for pointing this out. A correction
was posted on the discussion email list about this.
The symmetry element present is a centre of inversion through the centre of
the C-C single bond. The molecular point symmetry will be -1.
Comment: You have missed two of the symmetry elements
- see model answer for correct solutiom.
2 Marks
6.(i) Symmetry elements present are mirror
planes, glide planes and 2 fold
rotation axes.
Comment: Correct, though a little brief
3 Marks
(ii) Red object related to magenta objects:
Z - mirror
plane
J - glide
K - 2 fold rotation axis
3 Marks
(iii) K - no change of
handedness, same as the red object - 2 fold rotation does not involve any change
of handedness.
Z - change in handedness from the red object due to the mirror
plane.
J - change in handedness from the red object due to glide
plane.
3 Marks
(iv) Space group Pma2 therefore crystal system mm2.
Comment: Correct space group, but not the
correct crystal system: mm2 is the crystal class.
0 Marks
Comment: Overall, the assignment was done well,
though a little too brief in a few places