Effects of Structure & Bonding (AQA A Level Chemistry)

• 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⁺O^2- 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

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

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.