Structures: Simple Molecular, Giant Covalent and Metallic

Learning Objectives (OCR Spec 2.2.2)

By the end of this lesson you should be able to:

• Classify solids into four main structural types: simple molecular, giant covalent, giant metallic and giant ionic
• Describe the structure and bonding of diamond, graphite, silicon(IV) oxide and metals
• Explain the physical properties (m.p., hardness, conductivity, solubility) of each structure type
• Relate structure, bonding and properties throughout

Four Main Structures in Solids

graph TD
  A[Solid structures] --> B[Simple molecular]
  A --> C[Giant covalent]
  A --> D[Giant metallic]
  A --> E[Giant ionic]
  B -.-> B1[e.g. I2, ice, CO2, P4]
  C -.-> C1[e.g. diamond, graphite, SiO2]
  D -.-> D1[e.g. Na, Fe, Cu]
  E -.-> E1[e.g. NaCl, MgO - covered previously]

The first three are the focus of this lesson; giant ionic was covered in Lesson 6.

Simple Molecular Structures

A simple molecular solid consists of small, discrete molecules held together by intermolecular forces (London, dipole-dipole, hydrogen bonding). The covalent bonds within each molecule are strong, but the forces between molecules are weak.

Examples

• Iodine, I2 (solid at room temperature; purple crystals)
• Carbon dioxide, CO2 (dry ice)
• Ice, H2O
• Sulfur, S8
• Phosphorus, P4
• Buckminsterfullerene, C60

Properties

Property	Explanation
Low m.p./b.p.	Weak intermolecular forces easily overcome
Usually soft or crumbly	Weak attractions between molecules
Do not conduct electricity	No mobile charges (ions or delocalised electrons)
Poor thermal conductors	No free electrons
Often soluble in non-polar solvents (iodine in hexane)	Polarity matching
Polar molecules (water, ethanol) soluble in water	Form H bonds with water

Giant Covalent (Macromolecular) Structures

A giant covalent structure consists of a continuous network of atoms held together by covalent bonds throughout the entire solid. There are no discrete molecules; you could think of the whole crystal as "one giant molecule".

Diamond

• Each C atom is bonded to 4 others by covalent bonds in a tetrahedral arrangement
• Bond angle 109.5 degrees
• The result is a rigid 3D network

Properties:

• Very high melting point (~3700 degC) because you must break strong covalent bonds
• Extremely hard - the hardest natural substance known
• Does not conduct electricity - all 4 outer electrons on each C are in localised bonds, none are delocalised
• Insoluble in water and organic solvents (lattice cannot be broken easily)
• Good thermal conductor (vibrations transmit efficiently through the rigid lattice)

Uses: Drill bits, cutting and grinding tools, abrasives, jewellery

Graphite

• Each C atom is bonded to 3 others by covalent bonds, forming hexagonal layers
• Bond angle 120 degrees
• The fourth electron per carbon is delocalised over the layer
• Weak London forces hold layers together

Properties:

• High melting point (>3500 degC) - strong covalent bonds within layers
• Soft and slippery - layers slide over each other easily (weak London forces only)
• Conducts electricity along the planes (but not perpendicular to them) - delocalised electrons
• Insoluble in water and organic solvents
• Lower density than diamond (layers are widely spaced)

Uses: Pencil "lead", electrodes, lubricant, brushes in electric motors

Diamond vs Graphite Comparison

Property	Diamond	Graphite
Bonds per C	4	3
Structure	Tetrahedral network	Hexagonal layers
Delocalised electrons	None	Yes (1 per C)
Conductivity	Insulator	Good conductor (in plane)
Hardness	Hardest natural	Soft, slippery
Density	3.5 g cm-3	2.3 g cm-3
Melting point	~3700 degC	~3650 degC

Both are allotropes of carbon - the same element in different structural forms.

Silicon(IV) Oxide (Silica, SiO2)

• Structure similar to diamond: each Si bonded to 4 O, each O bonded to 2 Si, in a 3D tetrahedral network
• Found naturally as quartz and sand

Properties:

• Very high melting point (~1710 degC) due to strong Si-O covalent bonds throughout
• Hard (similar reason)
• Does not conduct electricity (no free electrons)
• Insoluble in water

Uses: Glass, silicon chips (after reduction), abrasives

Graphene (mentioned in some specs)

• Single layer of graphite: one atom thick
• Extremely strong (tensile strength 100 x that of steel per unit weight)
• Excellent conductor
• An emerging material in electronics

Giant Metallic Structures

A giant metallic structure consists of a lattice of positive metal ions in a "sea" of delocalised valence electrons. The metallic bond is the electrostatic attraction between the cations and the sea of mobile electrons.

Bonding Model