Key Concepts
- Allotropes are forms of the same element which exhibit different physical properties.
- Elements such as carbon, oxygen, phosphorus, tin and sulfur, display allotropy.
- The different physical properties displayed by allotropes of an element are explained by the fact that the atoms are arranged into molecules or crystals in different ways.
- Some allotropes of an element may be more chemically stable than others.
Allotropes of Oxygen
There are two main allotropes of oxygen, molecular oxygen (O2) and ozone (O3).
Both allotropes of oxygen are made up only of oxygen atoms, but they differ in the arrangement of the oxygen atoms.
O2 is a linear molecule while O3 is a bent molecule.
O2 and O3 have different physical properties such as colour, odour, melting and boiling point, density and solubility.
Some properties of the allotropes of oxygen are shown below:
| Property |
Oxygen (O2) |
Ozone (O3) |
|---|
| Structure |
O=O
linear |
bent |
|
| Colour |
colourless gas
pale blue liquid
pale blue solid |
pale blue gas
deep blue liquid
deep violet solid |
|
| Odour |
odourless |
sharp, pungent |
|
| Melting Point (oC) |
-219 |
-193 |
|
| Boiling Point (oC) |
-183 |
-111 |
|
| Density (20oC) |
1.3 g/L |
2.0 g/L |
|
| Solubility in Water |
slightly soluble |
more soluble that O2 |
|
| Chemical Stability |
stable |
decomposes to O2 easily |
|
| Uses |
common oxidiser |
sterilising agent
it is poisonous to many living things |
Allotropes of Carbon
The two most common, naturally occurring allotropes of carbon are graphite and diamond.
Both graphite and diamond are made up of carbon atoms, but the arrangement of atoms is different in each allotrope which results in different physical properties.
In particular, the presence of delocalised electrons in the structure of graphite results in it being soft and a good electrical conductor whereas diamond is very hard and an electrical insulator.
Some properties of graphite and diamond are shown below:
| Property |
Graphite |
Diamond |
|---|
| Structure |
Each carbon atom is bonded to 3 other carbon atoms in layers with delocalised electrons between the layers. |
Each carbon atom is bonded to 4 other carbon atoms in a 3-dimensional covalent network. All valence electrons are used in bonding. |
|
| Colour |
black |
colourless |
|
| Melting Point (K) |
sublimes at ~3500 |
sublimes at ~4000 |
|
| Electrical Conductivity |
good
delocalised electrons between the layers allow an electric current to pass through |
poor (an insulator)
no delocalised electrons to allow for the flow of electrical current |
|
| Hardness (Mohs Scale) |
1-2 (soft)
delocalised electrons allow the sheets to move over each other |
10
(hardest known natural mineral) |
|
| Chemical Stability |
stable |
decomposes slowly over time |
|
| Uses |
lubricant
because it is soft |
abrasive
because it is so hard |
Allotropes of Phosphorus
There are three allotropes of phosphorus; white, red and black.
Some properties of the allotropes of phosphorus are given below:
| Property |
White Phosphorus |
Red Phosphorus |
Black Phosphorus |
|---|
| Structure |
P4 molecules packed into a crystal |
Chains of P4 molecules
polymer |
Puckered layers of phosphorus atoms
polymer |
|
| Colour |
white |
red |
black |
|
| Chemical Stability |
least stable |
intermediate stability |
most stable |
Allotropes of Sulfur
Sulfur has several allotropes.
 -sulfur forms yellow, rhombic crystals out of 8-membered rings of sulfur atoms (S8).
 -sulfur forms yellow, monoclinic, needle-like crystals out of 8-membered rings of sulfur atoms (S8).
Plastic sulfur is yellow and made up of long chains of sulfur atoms. It reverts to S8 rings in time.
Allotropes of Tin
There are three allotropes of tin:
- Grey tin ( tin): a diamond-type lattice structure
- White tin (
 tin): body centred tetragonal structure
- brittle tin: rhombic structure
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