Lattice Structures (CIE A Level Chemistry)

• Most ionic, metallic and covalent compounds are crystalline lattice
• The ions, atoms or molecules are arranged in a regular and repeating arrangement

Giant ionic lattices

• An ionic bond is an electrostatic force between a positively charged metal (cation) ion and a negatively charged non-metal (anion) ion
  • The metal becomes positively charged as it transfers electrons to the non-metal becomes negatively charged

• Ionic compounds are arranged in giant ionic lattices (also called giant ionic structures)
• The type of lattice formed depends on the sizes of the positive and negative ions which are arranged in an alternating fashion
  • The ionic lattice of MgO and NaCl are cubic

Ionic lattices of the ionic compounds NaCl and MgO

The ionic lattices for NaCl and MgO are similar due to the ratio of 1 cation : 1 anion

 

General ionic lattice

A general ionic lattice shows the actual packing of the ions based on their relative size

Covalent lattices

• Covalent bonds are bonds between non-metals in which electrons are shared between the atoms
• Covalent compounds can be arranged in simple molecular or giant molecular lattices
  • Simple molecular lattices: Iodine, buckminsterfullerene (C₆₀) and ice
  • Giant molecular: silicon(IV) oxide, graphite and diamond

Simple molecular lattices

Ice, buckminsterfullerene and iodine all have different simple molecular lattices due to the different structures and intermolecular forces

Giant molecular lattices 

Giant molecular lattices have higher melting and boiling points because they require more energy to overcome the intramolecular and / or intermolecular forces

Metallic lattices

• Metals form giant metallic lattices in which the metal ions are surrounded by a ‘sea’ of delocalised electrons
• The metal ions are often packed in hexagonal layers or in a cubic arrangement

 

The layered structure of copper atoms

Copper cations are arranged in regular layers (the delocalised electrons are not shown in the diagram)