Crystals and Crystallisation (1/2) [Shankland] Flashcards
Explain the arrangement of crystalline materials
Molecules are arranged in a regular manner
In 3 dimension
Over long ranges
How can it be found out whether a material is crystalline or amorphous?
Expose samples of the material to X-ray radiation
Scattered radiation = crystalline
Single broad peak = amorphous
List the 4 forces which hold a solid together
Non-covalent, intermolecular forces:
Electrostatic interaction = strong, occurs between ions
Hydrogen bond = intermediate, between 2 permanent dipoles
Dipole-dipole interaction = weak, arising from permanent dipoles
Dispersion = occurs between all molecules, arising from transient dipoles
Name the 4 types of crystal arrangement
Ionic = electrostatic interactions Metallic = close-packed atoms Covalent = covalent bonding Molecular = van der Waals, hydrogen bonding and electrostatic interactions
Describe the arrangement of an ionic crystal
Held together by electrostatic interactions (e.g. Na+ & Cl-)
Regular, repeating pattern = long 3D order = crystalline
Describe the arrangement of a metallic crystal
Atoms are closely packed together
Atoms share electrons which are mobile, giving rise to conductivity
PICTURE
Describe the arrangement of a covalent crystal
Held together by covalent bonds
e.g diamond
Describe the arrangement of a molecular crystal
Held together by van der Waals, hydrogen bonding and electrostatic interactions
e.g. salt forms
How is a crystal described?
Molecular crystals consist of regularly arranged molecules in 3D extending (effectively) to infinity
“unit cell”
Which 3 bits of information about the unit cell is required in order to describe a crystal?
Size of the unit cell
Shape of the unit cell
Positions of atoms in the unit cell
(x, y, z dimensions)
Define: Polymorphs
Polymorphs are different 3D packing arrangements of the same object
How can polymorphs differ?
The size and shape of their unit cells
The location of the molecule inside their unit cells
