Allotropes Chemistry Tutorial

Key Concepts

• Allotropes are forms of the same element which exhibit different physical properties.
• Elements such as carbon, oxygen, phosphorus, tin and sulfur, display the property known as 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.

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Allotropes of Oxygen

There are two main allotropes of oxygen:

• A diatomic molecule made up of 2 oxygen atoms with the moelcular formula O₂ commonly referred to as molecular oxygen or dioxygen.
• A triatomic molecule made up of 3 atoms of oxygen with the molecular formula O₃ referred to as ozone.

Both allotropes of oxygen are made up only of oxygen atoms, but they differ in the arrangement of the oxygen atoms:

• Molecular oxygen (dioxygen), O₂, is a linear molecule.
• Ozone, O₃, is a bent molecule.

Molecular oxygen, O₂, and ozone, 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 in the table below:

	Allotropes of Oxygen
Property	Molecular 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 (°C)	-219	-193
Boiling Point (°C)	-183	-111
Density (20°C)	1.3 g L-1	2.0 g L-1
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:⁽¹⁾

• graphite
• 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 in the table below:

	Allotropes of Carbon
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 phosphorus
• red phosphorus
• black phosphorus

Some properties of the allotropes of phosphorus are given in the table below:

Allotropes of Sulfur

Sulfur has several allotropes:

• α-sulfur forms yellow, rhombic crystals out of 8-membered rings of sulfur atoms (S₈).
• γ-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 (α tin): a diamond-type lattice structure
• White tin (β tin): body centred tetragonal structure
• brittle tin: rhombic structure