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Unit 2: Properties of materials

Simple and Giant Structures — HIA International - Grade 8 Revision Notes & Topical Questions

5 min read 8 parts Lesson 12 of 12 3 quizzes

📚 HIA International - Grade 8 Hub / Topical Revision
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Lesson summary

Simple and giant structures — HIA INTERNATIONAL notes: Compare giant ionic, simple molecular and giant covalent structures and link structure to properties.Lattice — ions in sodium chloride form a giant regular structure; each Na+ is surrounded by six Cl−.Forces — strong electros…

  • Strong bonds
  • Weak forces
  • Low melting and boiling points
  • Conductivity
  • Macromolecule
  • Graphite uses

Part 1 of 6Giant ionic structures

Giant ionic structures
Lattice
  • Lattice ions in sodium chloride form a giant regular structure; each Na+ is surrounded by six Cl−.
Forces
  • Forces strong electrostatic forces act in all directions between oppositely charged ions.
Crystals
  • Crystals the regular pattern gives crystals a regular shape.

Part 2 of 6Simple molecular structures

Simple molecular structures
Examples
  • Examples oxygen, carbon dioxide and methane.
Strong bonds
  • Strong bonds covalent bonds inside the molecules are strong.
Weak forces
  • Weak forces intermolecular forces between molecules are weak.
Low melting and boiling points
  • Low melting and boiling points only a little energy is needed to overcome the weak forces.
Conductivity
  • Conductivity they do not conduct electricity.

Part 3 of 6Giant covalent structures

Giant covalent structures
Macromolecule
Macromolecule a large structure in which atoms are joined by strong covalent bonds, e.g. silicon dioxide.
Diamond
  • Diamond each carbon forms four strong covalent bonds in a rigid three-dimensional lattice,
    • making it the hardest material, used in cutting and drilling tools.
Graphite
  • Graphite each carbon forms three bonds in layers, with weak bonds between the layers, so layers slide and it is soft.
Graphite uses
  • Graphite uses pencil lead and lubricating moving parts.
Silicon dioxide
  • Silicon dioxide sand, hard with a melting point of 1610 °C.

Part 4 of 6Comparing properties

PropertyGiant ionicSimple molecularGiant covalent
Melting and boiling pointsVery highLowVery high
ReasonStrong forces between ionsWeak forces between moleculesMany strong covalent bonds
Conducts electricityOnly molten or dissolvedNoNo (graphite does conduct)

Part 5 of 6Conduction in ionic compounds

Solid
  • Solid ions are fixed in the lattice, so it does not conduct.
Molten or dissolved
  • Molten or dissolved the ions are free to move and carry charge.
Example data
  • Example data sodium chloride melts at 801 °C and boils at 1413 °C.
Metals
  • Metals a lattice of positive ions in a sea of electrons gives high melting points and conduction.

Part 6 of 6Identifying a structure

1
Check the melting point; low suggests simple molecular.
2
If high, test conductivity as a solid.
3
Conducts as a solid: metal (or graphite).
4
Conducts only when molten or dissolved: giant ionic.
5
No conduction and high melting point: giant covalent.

Key terms

TermMeaning
LatticeA giant regular arrangement of particles.
MacromoleculeA giant structure of atoms joined by covalent bonds.
Intermolecular forceA weak force between molecules.
Electrostatic forceThe attraction between oppositely charged ions.
GraphiteA soft form of carbon with layers that slide.
DiamondA very hard form of carbon.

Avoid these mistakes

1
Covalent substances always have low melting points — giant covalent substances have very high ones.
2
Weak bonds inside molecules explain low melting points — the weak forces are between molecules.
3
Solid ionic compounds conduct — they conduct only when molten or dissolved.
4
Diamond and graphite are different because of different atoms — both are carbon with different structures.

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