2: Solid state Chem Flashcards
Topic 2
Crystalline solids
atoms, ions or molecules of the material are arranged in a definite repeating pattern
The repeating patterns are fundamental properties and influence macroscopic behaviour
Metals, ions and small molecules typically for crystals
Amorphous solids
form when liquids freeze before the molecules can arrange into an orderly pattern
Lack ordered internal structure
Glasses
Non-crystalline materials
Large molecules or mixtures with restricted movement often form amorphous solids
Molecules that can form both amorphous and crystalline solids
Silicon dioxide can crystallise in several crystalline forms (quartz)
Rapid cooling forms amorphous fused silica
Types of crystalline solids
Ionic, metallic, covalent network, molecular
Ionic solids
made up of ions
positive and negatively charged ions
hard but brittle and shatter
high lattice energies
high m.p.
not conductive when solid
conductive when molten
Ionic solids
made up of ions
positive and negatively charged ions
hard but brittle and shatter
high lattice energies
high m.p.
not conductive when solid
conductive when molten
NaCl
Metallic solids
Atomic nuclei in a sea of delocalised electrons
metallic bonding
hard, malleable, don’t shatter
high thermal and electric conductivity
variable melting points
Copper
Covalent network solids
3D network of covalent bonds
non-metals
strong covalent bonds
very high m.p
Diamond
Molecular solids
neutral molecules held together by intermolecular forces
M.p varies based on intermolecular forces
small symmetrical non-polar molecules have weak attractive forces low m.p
molecules with permanent dipole higher m.p
H2O
Types of cubic unit cell
simple, face-centred, body-centred
Simple cubic unit cell
- Particles at 8 corners
- No central particle
- Spheres touch
- Not packed efficiently
- 48% empty
- Metallic polonium = only known example
- Each atom touching 6 others
- Coordination no. = 6
- PPUC = 8 x (1/8) = 1
Density of unit cell
Find number of particles in unit cell
PPUC
D = M/V
V = 2r^3
Find mass per particle
Molar mass/6 x 10^23 = mass per particle
D = M/V
Body-centred cubic unit cell
- Particles at all 8 corners
- 1 particle in the centre
- Centre particle touches all corners
- More efficiently packed than simple cubic
- 32% empty space
- Common structure for metals
- Lithium
- CN = 8. central atom touching 8 corner
- PPUC = (1)(1) + (8)(1/8) = 2
Face-centred cubic
- Particles at all 8 corners
- Particles at the centre of all 6 faces
- No central particle
- Corner spheres touch face spheres but not other corners
- Very efficiently packed
- 26% empty space
- Common for metals
- Al, Ca, Ni
- Copper
- CN = 12
- PPUC = (6)(1/2) + (8)(1/8) = 4
Copper as an antimicrobial surface
- copper shows contact killing effect
- copper surfaces kill pathogens
- likely due to release of ions leads to radical oxygen species that damage DNA and membrane
Hexagonal close packing
triangular holes
one layer offset from the other
74% packing efficiency
Magnesium
Cubic close packing
3 layer offset from each other
fcc
74% packing efficiency
Copper
Interstitial sites
- Smaller cations fit into holes
- Not all holes equal shape and size
- for every n particles there are n octahedral holes and 2n tetrahedral holes
Interstitial holes types
cubic hole
octahedral holes
tetrahedral holes
Cubic hole
a particle in this hole would touch 8 other particles
Octahedral hole
a particle in this hole would touch 6 other particles
Tetrahedral hole
a particle in this hole would touch 4 other particles
Cation and anion of similar size lattice
AB structure
- Caesium chloride (CsCl)
- Simple cubic lattice
- large anion fills cubic hole in centre
- NOT bcc AS ALL PARTICLES NOT IDENTICAL
- Anion:Cation = 1:1
- 1 CsCl/cell
Cations smaller than anions lattice AB structure
- NaCl
- FCC Na in octahedral holes
- 1:1
- alkali halides
- alkali hydrides
- monoxides
- monosulphides
Cation much smaller than anion lattice AB
- Zinc Blende
- ZnS
- fcc of sulfide/anion
- Zn in 50% of tetrahedral holes
- As charge dictates 1:1 but particles to tetrahedral holes are 1:2, only fill half tetrahedral holes
Fluorite structure (CaF2) AB2
- when 1:2 stoichiometry, all tetrahedral holes can be filled in fcc array
- Cations Ca2+ make up fcc
- F- fills holes
- ^ due to Ca:F = 1:2
- Ca in cubic environment - touch 8
- F in tetrahedral environment touch 4
Anti-fluorite AB2 lattice structure
- anions fill fcc spaces
- cation fills tetrahedral holes
- M2O - M = alkali metal
Rutile structure (TiO2) AB2 lattice structure
- Ti cations bcc tetragonal lattice
- Oxides in trigonal holes
- each Ti is octahedral environment
- SnO2, ZrO2
Rhenium trioxide structure (ReO3) AB3
- Re has simple cubic lattice
- O in between each Re along edges
- 1:3
- lots of unfilled space
Perovskite (CaTiO3) ABX3
- Same as ReO3 but with central cavity filled by additional element
- Ternary oxides ABO3
- Larger ion in gap
Network solids: Cristobalite (SiO2) AB2 network covalent solid
- resembles zinc blende structure
- fcc lattice of Si
- 50% tet holes filled by Si
- O between each Si atom
- Si CN = 4
- O CN = 2
- BeF2 similar structure
Most stable C allotrope under standard conditions?
Graphite.
stacked sheets of trigonal planar carbon form strong sigma network.
Delocalised pi electrons can conduct electricity
hexagonal solid state structure with ABAB layers.
Carbon allotrope under high pressure
Diamond
tetrahedrally arranged network
fcc lattice of C
with 50% of tetrahedral holes filled
CN = 4
C60
- Buckminsterfullerene
- not covalent network
- A molecule of C60 can form solid state structures with others
fcc unit cell with other C60 molecules
Defects in solid state structures
vacancies = atom/ion missing
interstitial impurity = atoms or ions fit into gaps
substitution impurity = atom/ion replaces on in the lattice if similar size or will distort if much larger
Doping of semiconductors
defects may introduce holes/extra valence electrons
may conduct electricity
Alloy
metal mixture of two or more pure elements
ductile, malleable, good conductors
