1.6.1 Evidence of Ions

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

Giant ionic lattices

• An ionic bond is an electrostatic force of attraction 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 which then becomes negatively charged
  • When an ionic compound is formed, the attraction between the ions happens in all directions

• 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

Electron density maps shown above show the likelihood of finding electrons in a region. This shows the electron density map for NaCl. The contours are lines of equal electron density. A = Na⁺ ions (smaller), B = Cl^- ions (larger). Between the ions the electron density falls to zero. 

 

General ionic lattice which shows the actual packing of the ions

Evidence 

• The behaviour of ionic substances during electrolysis is a clear piece of evidence for the existence of ions 
  • Positive ions in solution are attracted to the negative electrode
  • Negative ions in solution are attracted to the positive electrode

Set up of electrolysis 

• An example which is simple to see the separation is using copper(II) chromate(VI), CuCrO₄
• The solution contains
  • Cu²⁺ ions (blue)
  • CrO₄^2- ions (yellow)

• Overall the solution is an olive green colour, but when the solution undergoes electrolysis a blue colour appears around the negative electrode, and a yellow colour appears around the positive electrode
• This is because the Cu²⁺ ions are attracted to the negative electrode and their blue colour is observed and the CrO₄^2- ions are attracted to the positive electrode and their yellow colour is observed