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Allotropes

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.

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


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