Structural chemistry (probert) Flashcards
what is the simplest way an atom can be represented
just as a single atom for example polonium
define lattice
an array of regularly spaced points, which shows the repeating nature of the structure but not the actual form of the basic repeat unit
define crystal structure
the combination of the lattice and the contents of the repeat unit
define unit cell
the shape of the repeat unit, obtained by connecting adjacent lattice points; in two dimensions, this is a parallelogram
how is a unit cell defined
The unit cell is defined by 3 axes, and the angles between them
what is the unit cell in a simple cube
it is a simple cube so all axes are the same length and all angles are 90 degrees
many different unit cells could be chosen for convenience and convention which one do we pick
the one with the shortest sides and the angle closest to 90 degrees
which symmetry operations should we be aware of in the 3D crystalline solid state
Inversion: no effect (all 3D unit cell shapes have inversion symmetry, even when their contents do not)
Rotation: may force two unit cell lengths to be equal and/or some angles to be 90 or 120°; only C2 , C3 , C4 , C6 are possible
Reflection: forces some angles to be 90
what are fractional coordinates and why are they used
It is not convenient in solid state structures to give coordinates in absolute distances as they are not readily interpretable. Instead we use a system based on the fraction of the unit cell edges, so all coordinates are between 0 and 1 if in the same unit cell.
how do we simplify structural diagrams
In an attempt to simplify structural diagrams, projection representations are used to show the structure viewed down an axis with the coordinates of atoms given only for the out of plane positions
what does the solid state structure depends on
Atoms, ions and molecules are not points they have definitive sizes and a solid state structure will depend on how they can fit together.
compare the differences in packing with molecular solids, metals and alloys and ionic structures
For molecular solids, with covalent molecules held together by van der Waals interactions, the molecules tend to be packed together closely to maximise these interactions BUT strong repulsions or large thermal motion often restricts the packing efficiency.
Many metals and alloys, in which there are limited dominating interactions tend to pack in the most efficient ways, i.e. those in which the atoms are closely packed.
In addition to this many ionic structures can be understood in terms of closely packed anions (with some distortions), with the (almost always) smaller cations fitted into the small gaps between them
what is hexagonal close packed
Hexagonal close packed - a 2 layer repeat pattern ABABAB
This produces a structure with a unit cell having the shape of the hexagonal crystal system (a = b not c; a = b = 90°, g = 120°), the principal hexagonal axis being the stacking direction.
what is a face centred cubic
Face centred cubic - a 3 layer pattern ABCABCABC. It turns out that this pattern has cubic symmetry. The unit cell is a cube, with spheres at the corners and also in the middle of each face, so it is a face-centred cubic (fcc) or cubic close packing (ccp) arrangement. The directions perpendicular to close-packed layers are the body diagonals of the cube (which are all equivalent; there is not a unique direction of close packing, unlike hcp).
what is the problem with all structures
Even close-packed spheres have small gaps between them; the spheres themselves do not occupy all of the space, leaving quite large free voids/gaps. In both hcp and ccp arrangements, there are two kinds of gaps (or holes, as they are usually called), of different sizes. Because of their different surroundings, they are called octahedral holes and tetrahedral holes.
