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2.6Atoms, Elements and Compounds

Giant covalent structures — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions

5 min read 7 parts Lesson 6 of 7 1 quiz

📚 Cambridge IGCSE Chemistry 0620 Hub / Topical Revision
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2.6 Giant covalent structures — Notes — IGCSE Chemistry notes: Relate the structures of diamond, graphite and silicon(IV) oxide to their properties and uses.Giant covalent structure: billions of non-metal atoms joined by strong covalent bonds in a continuous network or layers.All…

  • Allotropes
  • Delocalised electron
  • Tetrahedral
  • There are no molecules and no intermolecular forces to overcome, as strong covalent bonds run throughout
  • All the bonds are identical and very strong
  • Very hard and dense, so it is used in cutting tools and drill bits

Section 1 of 7Giant covalent structures

1
Giant covalent structure: billions of non-metal atoms joined by strong covalent bonds in a continuous network or layers.
2
Allotropes: diamond and graphite are different structural forms of carbon,
  • which both contain only carbon atoms but have completely different properties.
3
Shared properties: very high melting points, because a lot of energy is needed to break the many strong covalent bonds, and insoluble in water.
4
There are no molecules and no intermolecular forces to overcome, as strong covalent bonds run throughout.

Section 2 of 7Diamond

1
Structure: each carbon atom forms four strong covalent bonds to four others, giving a rigid three-dimensional tetrahedral network.
2
All the bonds are identical and very strong.
3
Very hard and dense, so it is used in cutting tools and drill bits.
4
Hard is not the same as strong, as diamond is brittle and can be smashed with a hammer.
5
Does not conduct electricity, because all four outer electrons of each carbon are used in bonding, so there are no free charged particles.
6
Very high melting point: it sublimes at about 3550 °C.

Section 3 of 7Graphite

1
Structure: each carbon atom bonds to three others, forming flat layers of hexagonal rings.
2
Covalent bonds within a layer are very strong, and the layers are held together only by weak forces.
3
Conducts electricity: the fourth outer electron of each carbon is delocalised and moves along the layers carrying charge,
  • so it is used for electrodes.
4
Soft and slippery: the layers slide over each other because the forces between them are weak, so it is used as a lubricant and in pencils.
5
Very high melting point: many strong covalent bonds within the layers must be broken.
FeatureDiamondGraphite
Bonds per C atom4 (tetrahedral)3 (flat hexagonal layers)
Delocalised electronsNoneOne per C atom
HardnessVery hardSoft, slippery layers
Electrical conductivityDoes not conductConducts
Melting pointVery highVery high
UsesCutting tools, drill tipsElectrodes, lubricant, pencils

Section 4 of 7Silicon(IV) oxide (Supplement)

1
Silicon(IV) oxide (silicon dioxide, silica), SiO2, occurs naturally as sand and quartz.
2
It has a diamond-like giant covalent structure.
  • Each Si is bonded to 4 O atoms, forming a tetrahedron, and each O is bonded to 2 Si atoms.
  • The formula shows the ratio 1 Si : 2 O.
3
Properties: very hard, very high melting point, insoluble in water and does not conduct electricity,
  • because of the many strong covalent bonds and no intermolecular forces.
4
Uses: it is cheap and hard, so it is the abrasive on sandpaper and is used to line furnaces.

Section 5 of 7More from the specification

High melting points of giant covalent structures
  • High melting points of giant covalent structures covalent network bonds require immense thermal energy to break, providing a means of distinguishing non-conductors from metals.
Tetrahedral Lattice of Diamond
  • In diamond, each carbon atom is covalently bonded to four other carbon atoms in a rigid, three-dimensional tetrahedral lattice, imparting exceptional hardness, high melting point,
    • and electrical insulance.
Layered Allotropic Structure of Graphite
  • In graphite, each carbon atom is bonded to three others in planar hexagonal rings, leaving one delocalised electron per atom free to conduct electricity,
  • while weak van der Waals forces between layers allow them to slide easily.
Giant Covalent Network of Silicon(IV) Oxide
  • Quartz (SiO2) forms a vast three-dimensional giant covalent network where each silicon atom is bonded to four oxygen atoms and each oxygen to two silicons, explaining its hardness and elevated melting temperature.
Lattice
  • A regular, repeating three-dimensional arrangement of particles.
Oxygen
  • A gas that relights a glowing splint.
Mineral
  • A compound containing an element the body needs, such as iron or calcium.
Ionic compound
  • A compound made of positive and negative ions held together by electrostatic attraction.

Section 6 of 7Key terms

TermMeaning
Giant covalent structurea continuous network of atoms joined by strong covalent bonds.
Allotropesdifferent structural forms of the same element.
Delocalised electronan electron that is free to move through a structure.
Tetrahedralhaving each atom bonded to four others arranged around it.
Silicon(IV) oxideSiO2, a giant covalent compound found as sand and quartz.

Section 7 of 7Exam technique

1
Saying diamond has strong intermolecular forces: diamond has many strong covalent bonds and no molecules.
2
Saying graphite conducts because of free ions: it conducts because of delocalised electrons.
3
Saying graphite is soft because its covalent bonds are weak: the forces between the layers are weak, so layers slide.
4
Saying melting diamond overcomes weak forcesit requires breaking many strong covalent bonds.
5
Confusing hard with strongdiamond is hard but brittle.

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