Allotropy (HSC SSCE Chemistry): Revision Notes

Allotropy

What is allotropy?

Allotropy is when a single element exists in two or more physically different forms in the same physical state (solid, liquid, or gas). These different forms are called allotropes.

Allotropes are forms of the same element that:

• Exist in the same physical state
• Have distinctly different physical properties such as colour, density, hardness, and electrical conductivity

Common elements displaying allotropy

Several elements display allotropy, including:

• Carbon (diamond, graphite, buckminsterfullerene, carbon nanotubes)
• Phosphorus (white, red, black)
• Sulfur (rhombic, monoclinic, plastic)
• Oxygen ($\mathrm{O_2}$ and $\mathrm{O_3}$)
• Tin

Allotropes of carbon

Carbon shows the most striking differences between its allotropes. Let's explore the main carbon allotropes and understand how their structures determine their properties.

Diamond structure and properties

Diamond consists of carbon atoms, where each carbon atom is covalently bonded to four other carbon atoms in a tetrahedral arrangement. This creates a three-dimensional network structure.

Key structural features:

• Each carbon atom forms four strong covalent bonds
• Tetrahedral arrangement around each carbon atom
• Carbon atoms arranged in six-membered rings that are buckled (not flat)
• Three-dimensional covalent lattice structure
• All valence electrons are tied up in strong covalent bonds

Physical properties:

• Colourless and transparent
• Extremely hard (hardest naturally occurring material)
• Very high melting point (3550°C)
• Does not conduct electricity (no mobile electrons available)
• Not soluble in cyclohexane
• Brilliant appearance due to orderly atomic arrangement

Graphite structure and properties

Graphite is also a covalent network solid, but its structure is very different from diamond. Each carbon atom is bonded to only three other carbon atoms, forming flat, two-dimensional layers.

Key structural features:

• Each carbon atom forms three covalent bonds to other carbon atoms
• Planar (flat) layers of hexagonal six-membered rings
• Each carbon atom has one free valence electron
• Free electrons form a delocalised electron cloud (similar to metals)
• Weak intermolecular forces between layers (335 pm apart)
• Strong covalent bonds within layers (142 pm between atoms)

Physical properties:

• Black in colour
• Soft and slippery texture
• Used as a dry lubricant
• Conducts electricity
• Very high melting point (3974°C)
• Not soluble in cyclohexane

Buckminsterfullerene (C₆₀)

Buckminsterfullerene, often called 'buckyball', is a more recently discovered allotrope of carbon with the molecular formula $\mathrm{C_{60}}$.

Key structural features:

• Contains exactly 60 carbon atoms arranged in a spherical cage
• Structure resembles a football (soccer ball)
• Made up of 20 hexagonal rings and 12 pentagonal rings joined together
• Each carbon atom bonded to three other carbon atoms
• Bonds are not planar but form a curved surface
• Has some delocalised electrons but is less reactive than graphite

Physical properties:

• Yellow-brown colour
• Much lower melting point than diamond or graphite (550°C)
• Soluble in organic solvents like benzene, toluene, and cyclohexane (unlike diamond and graphite)
• Electrical conductivity properties are uncertain

Other fullerenes

Related molecules with formulae $\mathrm{C_{70}}$ and $\mathrm{C_{84}}$ have also been prepared. These are called fullerenes and are more egg-shaped than spherical. Each fullerene is an allotrope of carbon.

Carbon nanotubes

Carbon nanotubes are a synthetic allotrope of carbon that consists of small sheets of graphite rolled into cylinders.

Key features:

• Extremely strong
• High electrical conductivity
• Very small diameter (typically 1.3 nm)
• Can be very long (up to 1 cm, which is huge for a molecule)

Comparison of carbon allotropes

Allotrope	Melting point (°C)	Conducts electricity?	Soluble in cyclohexane?
Graphite	3974	Yes	No
Diamond	3550	No	No
Buckminsterfullerene ($\mathrm{C_{60}}$)	550	Unknown	Yes

Allotropes of phosphorus

There are three allotropes of phosphorus: white, red, and black. They differ significantly in their reactivity.

White phosphorus

Structure:

• Consists of $\mathrm{P_4}$ molecules packed into a crystal
• Each phosphorus atom is covalently bonded to three other phosphorus atoms in a tetrahedral arrangement

Properties:

• Very reactive
• Must be stored under water for safety
• Least stable allotrope

Red phosphorus

Structure:

• Consists of chains of $\mathrm{P_4}$ units
• One P—P bond in each $\mathrm{P_4}$ unit has 'opened out' to join with neighbouring units
• Forms a polymeric chain structure

Properties:

• Considerably less reactive than white phosphorus
• More stable than white phosphorus due to chain structure

Black phosphorus

Structure:

• Consists of puckered (buckled) layers of phosphorus atoms
• Each phosphorus atom bonded to three other phosphorus atoms
• Strong covalent bonds within layers
• Only weak forces between layers

Properties:

• Most stable allotrope
• Least reactive
• The layered structure provides greatest stability

Allotropes of sulfur

There are three allotropes of sulfur: rhombic, monoclinic, and plastic.

Rhombic and monoclinic sulfur

Structure:

• Both contain molecules with eight sulfur atoms ($\mathrm{S_8}$)
• Sulfur atoms joined by single covalent bonds in a ring structure
• The difference is only in how the $\mathrm{S_8}$ molecules are packed in the crystals

Properties:

• Similar chemical reactions (because they have the same molecular structure)
• Only small differences in physical properties

Plastic sulfur

Structure:

• Consists of long chains of sulfur atoms singly bonded to each other
• Non-crystalline (amorphous) structure

Properties:

• Rubbery texture
• Slowly reverts back to $\mathrm{S_8}$ rings on standing

Allotropes of oxygen

Oxygen exists in two allotropic forms:

Oxygen gas ($\mathrm{O_2}$)

Structure:

• Diatomic molecules
• Contains two oxygen atoms bonded together

Properties:

• Boiling point: −183°C
• Essential for respiration

Ozone ($\mathrm{O_3}$)

Structure:

• Triatomic molecules
• Contains three oxygen atoms bonded together

Properties:

• Poisonous gas
• Boiling point: −111°C (considerably higher than oxygen)
• Important in Earth's upper atmosphere where it absorbs harmful UV radiation

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