Giant Covalent Structures

Section 1 of 12What a giant covalent structure is

What a giant covalent structure is

Giant covalent structure

• Giant covalent structure billions of non-metal atoms each joined to neighbours by covalent bonds in a lattice.

Allotropes

• Allotropes diamond and graphite are different forms of carbon with different bonding arrangements.
• Same atoms, different properties
  • both contain only carbon, but they are physically very different.

Section 2 of 12Structure of diamond

Structure of diamond

Bonding

• Bonding each carbon atom is bonded to four others in a tetrahedral arrangement.

Bonds

• Bonds all identical, very strong, with no intermolecular forces.
• Electrons
  • all four outer electrons are held in bonds.

Section 3 of 12Structure of silicon(IV) oxide

Structure of silicon(IV) oxide

Formula

• SiO₂, also called silica, occurring naturally as sand and quartz.

Bonding

• Bonding each silicon bonds to four oxygens and each oxygen bonds to two silicons.
• Shape
  • a tetrahedron of one silicon and four oxygens, as in diamond.

Section 4 of 12Structure of graphite

Layers

• Layers each carbon bonds to three others, forming layers of hexagons.

Delocalised electrons

• Delocalised electrons one free electron per carbon is delocalised between the layers.
• Forces between layers
  • only weak intermolecular forces hold the layers together.
• Bonds within layers
  • strong covalent bonds.

Section 5 of 12Properties and uses of diamond

Does not conduct

• Does not conduct all outer electrons are in bonds, so no charge can move.

Very high melting point

• Very high melting point a large amount of energy is needed to break the strong lattice of bonds.

Extremely hard and dense

• Extremely hard and dense each carbon is bonded to four others.

Uses

• Uses jewellery, and tips of cutting discs and heavy-duty drill bits for brick and concrete.

Section 6 of 12Properties and uses of graphite

Conducts electricity

• Conducts electricity delocalised electrons move and carry charge.

Very high melting point

• Very high melting point strong covalent bonds within the layers.

Soft and slippery

• Soft and slippery layers slide over each other because of weak forces between them.

Density

• Density lower than diamond.

Uses

• Uses pencils, industrial lubricant in engines and locks, and unreactive electrodes for electrolysis.

Pencil lead

• Pencil lead it is graphite, not the metal lead; the name graphite comes from Latin for writing.

Section 7 of 12Comparing diamond, graphite and silicon(IV) oxide

Substance	Bonding	Electrical conductivity	Hardness	Melting point
Diamond	Each C bonded to 4 others	None	Extremely hard	Very high
Graphite	Each C bonded to 3 others in layers	Conducts	Soft, slippery	Very high
Silicon(IV) oxide	Si to 4 O, O to 2 Si	None	Very hard	Very high

Silica uses

• Silica uses cheap and natural, so used for sandpaper and to line furnaces.

Other properties

• Other properties insoluble in water.
• Cause
  • many strong covalent bonds and no intermolecular forces.

Section 8 of 12Why the structures give the properties

Why the structures give the properties

Diamond

• Diamond strong bonds in every direction make it hard with a very high melting point, and no free electrons make it an insulator.

Graphite

• Graphite strong bonds within layers give a high melting point, while weak forces between layers make it soft and slippery.

Conductivity

• Conductivity delocalised electrons between the layers carry charge.
• Lubricant
  • layers slide easily over each other.

Silicon(IV) oxide

• Silicon(IV) oxide the giant structure of strong bonds makes it hard, high melting and non-conducting.

Link

• Link its structure is similar to diamond, so its properties are similar.

Section 9 of 12Answering exam questions on giant structures

1

Name the type of structure: giant covalent.

2

State the particles involved and the bonds between them.

3

Explain the property by referring to the strong bonds, free electrons or weak forces.

4

Link the property to a use.

Answering exam questions on giant structures

Common trap

• Common trap do not say that bonds break when graphite slides;
  • the layers move apart because the weak forces between them are overcome.

Hard and brittle

• Hard and brittle hardness refers to resisting scratches, not to being unbreakable.

Section 10 of 12Worked examples

1. Explain why diamond is very hard but graphite is soft. [3]3 marks

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2. Explain why graphite conducts electricity but diamond does not. [2]2 marks

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3. Explain why silicon(IV) oxide has a very high melting point. [2]2 marks

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4. State a use of diamond and a use of graphite, each linked to a property. [2]2 marks

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Section 11 of 12Key terms

Term	Meaning
Allotropes	Different forms of the same element in the same state.
Giant covalent structure	A huge lattice of atoms joined by covalent bonds.
Delocalised electron	An electron free to move through a structure.
Tetrahedron	A shape with four bonds pointing to the corners, as in diamond.
Silicon(IV) oxide	SiO2, a giant covalent compound found as sand and quartz.

Section 12 of 12Exam technique

1

Confusing hard with strong — diamond is hard but brittle and can be smashed by a hammer.

2

Saying graphite has weak bonds — the bonds in layers are strong; only the forces between layers are weak.

3

Mixing pencil lead with the metal lead — pencil lead is graphite.

4

Saying giant covalent structures contain molecules — they have a continuous lattice of atoms.