Solid State Chemistry

Question 1

Solid

Answer 1

State of matter in which the constituent particles are arranged so that shape and volume are stable

Question 2

Solid state chemistry

Answer 2

The study of preparation, structure and properties of solid materials.

Question 3

Molecular solids

Answer 3

• Made from molecules
• Covalently bonded
• Intermolecular bonds are much weaker (H bonds, dipole, london dispersion)
• Soft materials
• Low mps

Question 4

Examples of molecular solids

Answer 4

• Iodine (I2)
• Sulfur (S8)
  Both are solid at room temperature and sublime easily to their gaseous forms due to weak IM forces

Question 5

Colvalet network solids

Answer 5

• Formed of infinitely, covalently bonded atoms.
• Can be formed by both elements and compounds
• High mps and bps
• Hard and brittle materials

Question 6

Examples of covalent network solids

Answer 6

• Carbons (graphite, diamond etc.)
• Silicon dioxide (SiO2) (quartz, alpha crisobalite are polymorphs)

Question 7

Polymorph

Answer 7

Different solid state structures of the same compound

Question 8

Metallic solids

Answer 8

• 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

Question 9

Examples of metallic solids

Answer 9

• Copper (Cu) (ccp)
• Titanium (Ti) (hcp)
• Iron (Fe) (bcc)

Question 10

Ionic solids

Answer 10

• 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

Question 11

Example of ionic solid

Answer 11

Na+Cl-

Question 12

Crystal

Answer 12

A solid consisting of a regular and repeating array of atoms, molecules or ions.

Question 13

Crystal unit cell

Answer 13

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.

Question 14

Symmetries displayed by crystals

Answer 14

• Mirror
• Rotational
• Inversion
• Rotary-inversion
• Translational

Question 15

Determining crystal structures

Answer 15

X-ray crystallography

Question 16

X-ray crystallography

Answer 16

• 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.

Question 17

Crystal packing

Answer 17

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.

Question 18

Cubic close packing (CCP)

Answer 18

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)

Question 19

Hexagonal close packing (HCP)

Answer 19

In 3 layers, the spheres of the third layer would sit directly above layer 1 spheres.

Question 20

Cubic unit cells

Answer 20

In CCP, all vector lengths are equal and all angles are 90 degrees. Seen as a “space filling model”.

Question 21

Hexagonal unit cell

Answer 21

In HCPs, the vector lengths are a = b ≠ c and the γ angle is 120 degrees.

Question 22

Body centred cubic cell (BCC)

Answer 22

Atoms on all vertices, plus one atom in the centre of the cell.

Question 23

Primitive cubic cell

Answer 23

Atoms only on the vertices

Question 24

Coordination numbers in CCP and HCP

Answer 24

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

Question 25

Coordination numbers in BCC

Answer 25

All atoms have CN of 8 - central atom is in contact with 8 atoms on the vertices

Question 26

Coordination number in Primitive

Answer 26

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.

Question 27

Interstitial sites

Answer 27

Gaps between closely packed atoms.

Question 28

Octahedral sites

Answer 28

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.

Question 29

Tetrahedral sites

Answer 29

Lie between a triangle of atoms in the row above, and sits directly above another atom. No of tetrahedral sites in a structure: 2N.

Question 30

Packing efficiency

Answer 30

A quantitative measure of how well atoms are packed together: (vol of atoms in unit cell/total volume of the unit cell) x 100

Question 31

Cell projection diagrams

Answer 31

2D diagrams of a unit cell viewed from above.

Question 32

Packing efficiency

Answer 32

(Volume of atoms in a unit cell/total volume of unit cell) x 100

Question 33

How to find the contribution of each atom to a unit cell

Answer 33

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.

Question 34

Total number of atoms in one unit cell

Answer 34

N = (No of atoms on vertices)(contribution) + (No of atoms on faces)(contribution)

Question 35

Total volume of the unit cell

Answer 35

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)

Question 36

Estimating density of solids

Answer 36

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.

Question 37

Intensive properties

Answer 37

Bulk properties that are independent of the size of the system. Density is an example.

Question 38

Total mass

Answer 38

Total number of atoms x the molar mass of the element.

Question 39

Density equation

Answer 39

Mass/volume (remember to convert to g and cm3

Question 40

Binary solids

Answer 40

- 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

Question 41

Structures based on CCP

Answer 41

- 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

Question 42

Structures based on HCP

Answer 42

- 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.

Question 43

Structures not based on close packing

Answer 43

- 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.