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.
what are octahedral holes
small octahedral holes between two close packed layers. around there are 3 spheres in the layer below and 3 in the layer above hence octahedral
what are tetrahedral holes
small tetrahedral holes between two close packed layers. around there are 3 spheres in one layer and 1 sphere in the other layer.
there are two groups of tetrahedral holes each lies directly above or below a sphere in an adjacent atom
how efficient is the primitive cubic
62.4%
how efficient is the body centred cubic
68%
how efficient is the cubic close packed and hexagonal close packed
74%
what are the maximum size of the holes for both octahedral and tetrahedral gaps
roct = 0.414 Rcp
rtet = 0.225 Rcp
is the structure of a metal fixed
The structure of a material is not necessarily fixed and unchangeable sometimes the same material may have different structures under different conditions
Many metals that are close packed under normal conditions will tent towards the less efficiently packed bcc structures at higher temperatures.
define ionisation energy
the energy required to remove the highest energy electron completely from an atom in the gas phase
when is the ionisation energy positive
the IE is positive for all neutral atoms, and for all all cations
describe successive ionisation energies
For any element, successive IEs increase (removal of 2nd, 3rd,… electrons); there is a huge jump when all valence electrons have been removed and the first core electron is taken next.
define electron affinity
the ease with which an electron can be added to an atom or ion
by definition electron affinity is the same as IE of that species with one extra electron
x-(g) -> x(g)+e-
what is electron gain
the negative of the electron affinity
X(g) +e- -> X-(g)
describe the sizes of cations and anions in comparison to the parent atom
Cations are smaller than their parent atoms.
Anions are larger than their parent atoms.
describe the trends of ionic radii
Ionic radii increase with greater negative charge, decrease with greater positive charge.
Almost all simple cations are smaller than almost all simple anions (those based on one atom). Ionic radii can vary with electron configuration (for transition metal cations), and with coordination number, for
the same ion.
describe the periodic table trends in terms of both IE and ionic radii
Across a period:
IEs rise sharply
ionic radii decrease
Down a group:
IEs fall slowly
ionic radii increase
