Solid State Chemistry Flashcards
Topic 6 - Lauren Hatcher
Solid
State of matter in which the constituent particles are arranged so that shape and volume are stable
Solid state chemistry
The study of preparation, structure and properties of solid materials.
Molecular solids
- Made from molecules
- Covalently bonded
- Intermolecular bonds are much weaker (H bonds, dipole, london dispersion)
- Soft materials
- Low mps
Examples of molecular solids
- Iodine (I2)
- Sulfur (S8)
Both are solid at room temperature and sublime easily to their gaseous forms due to weak IM forces
Colvalet network solids
- Formed of infinitely, covalently bonded atoms.
- Can be formed by both elements and compounds
- High mps and bps
- Hard and brittle materials
Examples of covalent network solids
- Carbons (graphite, diamond etc.)
- Silicon dioxide (SiO2) (quartz, alpha crisobalite are polymorphs)
Polymorph
Different solid state structures of the same compound
Metallic solids
- Cations in a sea of delocalised electrons
-Strong bonds due to forces of attraction between ions and electrons - High mps and bps
- Only valence electrons contribute towards bonding
- Highly conductive as delocalised electrons can move and carry charge
Examples of metallic solids
- Copper (Cu) (ccp)
- Titanium (Ti) (hcp)
- Iron (Fe) (bcc)
Ionic solids
- Formed of cations and anions
- Strong electrostatic interactions
- Elements with lower ionisation energies are more likely to form ionic compounds
- High mps and bps
- Hard and brittle
- Conductive when molten or in solution
Example of ionic solid
Na+Cl-
Crystal
A solid consisting of a regular and repeating array of atoms, molecules or ions.
Crystal unit cell
The smallest repeating unit of a crystal. Described by vector lengths (a, b and c) and the angles between them (α, β, and γ). When the unit cell is translated in three directions it generates the full crystal structure.
Symmetries displayed by crystals
- Mirror
- Rotational
- Inversion
- Rotary-inversion
- Translational
Determining crystal structures
X-ray crystallography
X-ray crystallography
- X-ray source: provides a beam that is directed towards a crystal
- Crystal: diffracts the beam as the gaps between atoms are of a similar order to the wavelength of the x-rays
- Detector: bright spots (maxima) and gaps (minima) are produced in a diffraction pattern. The pattern is mathematically related to the structure of the crystal that produced it and can be used to determine the structure.
Crystal packing
Atoms in crystals always endeavour to pack together as closely as possible - minimal gaps. When there are multiple close-packed layers, they align in a way that the spheres of one layer sit in the gaps of another.
Cubic close packing (CCP)
If there were 3 layers, the spheres of the third layer would sit directly above ayer 1 gaps. Also known as face centred cubic (FCC)
Hexagonal close packing (HCP)
In 3 layers, the spheres of the third layer would sit directly above layer 1 spheres.
Cubic unit cells
In CCP, all vector lengths are equal and all angles are 90 degrees. Seen as a “space filling model”.
Hexagonal unit cell
In HCPs, the vector lengths are a = b ≠ c and the γ angle is 120 degrees.
Body centred cubic cell (BCC)
Atoms on all vertices, plus one atom in the centre of the cell.
Primitive cubic cell
Atoms only on the vertices
Coordination numbers in CCP and HCP
All atoms have a CN of 12
- 6 atoms in contact in the same layer
- 3 atoms in contact in the layer below
- 3 atoms in contact in the layer above
Coordination numbers in BCC
All atoms have CN of 8
- central atom is in contact with 8 atoms on the vertices
Coordination number in Primitive
All atoms have CN of 6
- An atom on a vertex has one atom above and below, and 4 atoms in the same plane that are in contact.
Interstitial sites
Gaps between closely packed atoms.
Octahedral sites
Lie between a triangle of atoms in the row above and a triangle of atoms in the row below. No of octahedral sites in a structure: N.
Tetrahedral sites
Lie between a triangle of atoms in the row above, and sits directly above another atom. No of tetrahedral sites in a structure: 2N.
Packing efficiency
A quantitative measure of how well atoms are packed together:
(vol of atoms in unit cell/total volume of the unit cell) x 100
Cell projection diagrams
2D diagrams of a unit cell viewed from above.
Packing efficiency
(Volume of atoms in a unit cell/total volume of unit cell) x 100
How to find the contribution of each atom to a unit cell
Find the number of unit cells an atom is part of e.g. if an atom on a vertex also exists in 7 other unit cells, it has a contribution of 1/8.
Total number of atoms in one unit cell
N = (No of atoms on vertices)(contribution) + (No of atoms on faces)(contribution)
Total volume of the unit cell
Using the radius to find the diagonal length of the cell projection, use Pythagoras’ theorem to determine a as a function of r (this will change depending on which unit cells)
Estimating density of solids
Need to know the volume of the unit cell, and the total number of atoms in the unit cell, in order to calculate total mass.
Intensive properties
Bulk properties that are independent of the size of the system. Density is an example.
Total mass
Total number of atoms x the molar mass of the element.
Density equation
Mass/volume (remember to convert to g and cm3
Binary solids
- Solids comprised of two different elements
- Some can be covalently bonded or held together through IMF
- Many are ionic compounds
- Based on close packed structures: close packed array of ions of one element, with ions of the other element located in a proportion of the interstitial sites
Structures based on CCP
- Sodium Chloride (NaCl): CCP array of Cl- ions, with Na+ cations in the octahedral interstitial sites
- Fluorite (CaF2): CCP Ca2+, F- in tetrahedral sites
- Spheralite (ZnS): CCP S2-, Zn2+ in half the tetrahedral sites
- Cadmium chloride (CdCl2): CCP Cl-, Cd2+ in half of the octahedral sites
Structures based on HCP
- Nickel arsenide (NiAs): HCP array of As2-, Ni2+ in octahedral sites
- Wurzite (ZnS): HCP array of S2- anions, with Zn2+ in half the tetrahedral sites.
- Cadmium iodide (CdI2): HCP array of I- anions with Cd2+ in half the octahedral holes.
Structures not based on close packing
- Rutile (TiO2): distorted HCP, Ti4+ cations with O2- anions in half the octahedral sites.
- Caesium chloride (CsCl): primitive cubic packing of Cs+ cations, with Cl- anions in interstitial sites.
- Perovskite (CatIO3): primitive packing of Ca2+ ions, O2- in faces and Ti4+ in the centre of the cell.
- Spinel (MgAl2O4): CCP array of O2- cations, Mg2+ in 1/8 of tetrahedral sites and Al3+ in 1/2 the octahedral sites
- Inverse spinel (Mg2TiO4): CCP array of O2- cations with Ti4+ in 1/2 octahedral holes, Mg2+ in 1/2 octahedral sites and all of the tetrahedral sites.
