AQA A Level Chemistry

Revision Notes

1.4.10 Effects of Structure & Bonding

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Effects of Bonding & Structure on Physical Properties

  • Different types of structure and bonding have different effects on the physical properties of substances such as their melting and boiling points, electrical conductivity and solubility

Ionic bonding & giant ionic lattice structures

  • Ionic compounds are strong
    • The strong electrostatic forces in ionic compounds keep the ions strongly together

  • They are brittle (meaning ionic crystals can split apart easily)
  • Ionic compounds have high melting and boiling points
    • The strong electrostatic forces between the ions in the lattice act in all directions and keep them strongly together
    • Melting and boiling points increase with charge density of the ions due to the greater electrostatic attraction of charges
    • For example, Mg2+O2- has a higher melting point than Na+Cl-

  • Ionic compounds are soluble in water as they can form ion - dipole bonds
  • Ionic compounds only conduct electricity when molten or in solution
    • When molten or in solution, the ions can freely move around and conduct electricity
    • In the solid state they’re in a fixed position and unable to move around

Metallic bonding & giant metallic lattice structures

  • Metallic structures are malleable
    • When a force is applied, the metal layers can slide
    • The attractive forces between the metal ions and electrons act in all directions
    • So when the layers slide, the metallic bonds are re-formed
    • The lattice is not broken and has changed shape

  • Metallic lattices are strong and hard
    • Due to the strong attractive forces between the metal ions and delocalised electrons

  • Metals have high melting and boiling points
  • Pure metals are insoluble in water
  • Metals can conduct electricity when in the solid or liquid state
    • As both in the solid and liquid state there are mobile electrons which can freely move around and conduct electricity

States of Matter Metals Malleable, downloadable AS & A Level Chemistry revision notes

Metals are malleable as the layers can slide over each without breaking the attraction

Covalent bonding & simple covalent lattice structures

  • Simple covalent lattices have low melting and boiling points
    • These compounds have weak intermolecular forces between the molecules
    • Only little energy is required to break the lattice

  • Most compounds are insoluble with water
    • Unless they are polar and can form hydrogen bonds (such as sucrose)

  • They do not conduct electricity in the solid or liquid state as there are no charged particles
    • Some simple covalent compounds do conduct electricity in solution, but this is a reaction with the water than produces ions such as HCl which forms H+ and Cl- ions

Covalent bonding & giant covalent lattice structures

  • Giant covalent lattices have very high melting and boiling points
    • These compounds have a large number of covalent bonds linking the whole structure
    • A lot of energy is required to break the lattice

  • The compounds can be hard or soft
    • Graphite is soft as the forces between the carbon layers are weak
    • Diamond and silicon(IV) oxide are hard as it is difficult to break their 3D network of strong covalent bonds

  • Most compounds are insoluble with water
  • Most compounds do not conduct electricity however some do
    • Graphite has delocalised electrons between the carbon layers which can move along the layers when a voltage is applied
    • Diamond and silicon(IV) oxide do not conduct electricity as all four outer electrons on every carbon atom are involved in a covalent bond so there are no freely moving electrons available

Characteristics of Different Compound Structure Types Table

Characteristics of different compound structure types table, downloadable AS & A Level Chemistry revision notes

Worked example

Bonding & structure

The table below shows the physical properties of substances X, Y and Z

Worked Example - Bonding and Structure, downloadable IB Chemistry revision notes

Which one of the following statements about X, Y and Z is completely true?

Statement 1: X has a giant ionic structure, Y has a giant molecular structure, Z is a metal

Statement 2: X is a metal, Y has a simple molecular structure, Z has a giant molecular structure

Statement 3: X is a metal, Y has a simple molecular structure, Z has a giant ionic structure

Statement 4: X has a giant ionic structure, Y has a simple molecular structure, Z is a metal

Answer

The correct answer is Statement 4

    • The relatively high melting point, solubility in water and electrical conductivity when molten suggest that X is a giant ionic structure.
    • The low melting point of Y suggests that little energy is needed to break the lattice which corresponds to a simple molecular structure. This is further supported by the low electrical conductivity and its being almost insoluble in water.
    • Compound Z has a very high melting point which is characteristic of either metallic or giant molecular lattices, however since it conducts electricity, compound Z must be a giant metallic lattice.

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