Unit cells Flashcards
What is a microscopic structure?
A structure that is so small it can only be viewed under a microscope
The microscopic structure often determines its macroscopic properties
What is a macroscopic structure?
A structure that is visible to the naked eye
What is a monocrystal?
This is relating to or containing a single crystal
What is a polycrystalline structure?
A solid consisting of many crystalline parts that are randomly orientated with respect to each other
What is a unit cell?
The smallest repeat unit that describes the entire lattice
The structure of a crystal is derived from a basic building block (unit cell) which repeats throughout the entire structure
What is a lattice?
A regular, periodic array of points in space
There may not be any atoms at these points
A lattice is a regular extended structure
What are lattice parameters?
These are the dimensions of the unit cell the dimensions of the unit cell (1-3)
You also need to define (1-3) angles for non orthogonal cells
What types of solid are there?
Molecular
Covalent
Metallic
Ionic
Describe a molecular solid
There are strong covalent bonds between the atoms but the Intermolecular interactions are weak
Why can molecular solids easily sublime to a vapour?
They have weak interactions between the molecules and sublimation only involves breaking intermolecular interactions (the actual molecule remains intact)
The vapour contains the intact molecules
Describe a covalent solid
There is directional bonding from orbital overlap
A covalent network is formed where each atom forms covalent bonds to other atoms
What is a covalent network structure?
atoms can form covalent bonds to many other atoms
This leads to the formation of an infinite structure known as a covalent network structure
Why do covalent solids have high melting and boiling points?
Covalent bonds need to be broken to convert the solid into a liquid or gas
Describe a metallic solid
The electrons are delocalised and metal cations are held together by these delocalised electrons
Why do metallic solids conduct electricity?
In a metal, the valence electrons are delocalised so it can conduct electricity when a potential difference is applied
Describe an ionic solid
The atoms are ionised which causes them to have electrostatic interactions with other ions
This holds them together in a lattice
Electrostatic attractions between the oppositely charged ions hold an ionic solid together
What is meant by regular spacing?
Regular spacing means the positions of the atoms will be predictable
The unit cell enables us to predict the rest of the lattice
What is an allotrope?
Allotropes are different forms of the same element
Describe the properties of an ionic solid
They generally have high melting and boiling points
They conduct electricity in solution but not in the solid phase
What is the unit cell defined in terms of?
Each unit cell is defined in terms of shape, size, symmetry and number of atoms within the unit cell
Describe the properties of a unit cell
All unit cells of same structure will have same area
Not necessarily 2D or 3D- represents lattice symmetry - simplest symmetry
- shows position of neighbours
- can only move unit cell one unit up, down or across
- they are the smallest volume they can be
What are the allotropes of carbon?
Allotropes of carbon include diamond, graphite and Buckminsterfullerene
Which unit cell would be most useful?
The more useful unit cell would be one that shows the neighbours of the atoms
This makes it easier to determine structure and distance between the atoms
What structures do diamond and graphite bond form?
They both form covalent network structures
Their properties depend on their structures
In diamond, each carbon has a tetrahedral environment
What does Buckminsterfullerene consist of?
It consists of discrete C60 molecules that form a molecular solid
Why is diamond one of the hardest materials?
The C-C bonds are formed from the interaction of sp3 hybrid orbitals on the carbon atoms which are very strong
Describe the structure and bonding in graphite
Graphite forms a layer structure which has strong C-C bonds within layers but weaker interactions between the layers
The carbon atoms are in a trigonal planar environment which has sp2 hybridisation
How are the C-C bonds in graphite formed?
The C-C bonds are formed from the interactions between the sp2 hybridised and the unhybridised p orbitals interacting to form a pi system that is delocalised across the whole layer
What is a polymorph?
Different crystal structures of the same compound
