Unit cells

Question 1

What is a microscopic structure?

Answer 1

A structure that is so small it can only be viewed under a microscope
The microscopic structure often determines its macroscopic properties

Question 2

What is a macroscopic structure?

Answer 2

A structure that is visible to the naked eye

Question 3

What is a monocrystal?

Answer 3

This is relating to or containing a single crystal

Question 4

What is a polycrystalline structure?

Answer 4

A solid consisting of many crystalline parts that are randomly orientated with respect to each other

Question 5

What is a unit cell?

Answer 5

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

Question 6

What is a lattice?

Answer 6

A regular, periodic array of points in space
There may not be any atoms at these points
A lattice is a regular extended structure

Question 7

What are lattice parameters?

Answer 7

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

Question 8

What types of solid are there?

Answer 8

Molecular
Covalent
Metallic
Ionic

Question 9

Describe a molecular solid

Answer 9

There are strong covalent bonds between the atoms but the Intermolecular interactions are weak

Question 10

Why can molecular solids easily sublime to a vapour?

Answer 10

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

Question 11

Describe a covalent solid

Answer 11

There is directional bonding from orbital overlap

A covalent network is formed where each atom forms covalent bonds to other atoms

Question 12

What is a covalent network structure?

Answer 12

atoms can form covalent bonds to many other atoms

This leads to the formation of an infinite structure known as a covalent network structure

Question 13

Why do covalent solids have high melting and boiling points?

Answer 13

Covalent bonds need to be broken to convert the solid into a liquid or gas

Question 14

Describe a metallic solid

Answer 14

The electrons are delocalised and metal cations are held together by these delocalised electrons

Question 15

Why do metallic solids conduct electricity?

Answer 15

In a metal, the valence electrons are delocalised so it can conduct electricity when a potential difference is applied

Question 16

Describe an ionic solid

Answer 16

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

Question 17

What is meant by regular spacing?

Answer 17

Regular spacing means the positions of the atoms will be predictable
The unit cell enables us to predict the rest of the lattice

Question 18

What is an allotrope?

Answer 18

Allotropes are different forms of the same element

Question 19

Describe the properties of an ionic solid

Answer 19

They generally have high melting and boiling points

They conduct electricity in solution but not in the solid phase

Question 20

What is the unit cell defined in terms of?

Answer 20

Each unit cell is defined in terms of shape, size, symmetry and number of atoms within the unit cell

Question 21

Describe the properties of a unit cell

Answer 21

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

Question 22

What are the allotropes of carbon?

Answer 22

Allotropes of carbon include diamond, graphite and Buckminsterfullerene

Question 23

Which unit cell would be most useful?

Answer 23

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

Question 24

What structures do diamond and graphite bond form?

Answer 24

They both form covalent network structures
Their properties depend on their structures
In diamond, each carbon has a tetrahedral environment

Question 25

What does Buckminsterfullerene consist of?

Answer 25

It consists of discrete C60 molecules that form a molecular solid

Question 26

Why is diamond one of the hardest materials?

Answer 26

The C-C bonds are formed from the interaction of sp3 hybrid orbitals on the carbon atoms which are very strong

Question 27

Describe the structure and bonding in graphite

Answer 27

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

Question 28

How are the C-C bonds in graphite formed?

Answer 28

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

Question 29

What is a polymorph?

Answer 29

Different crystal structures of the same compound