Ionic Crystals (College Board AP Chemistry)

• Ionic crystals are arranged in a lattice structure of positive and negative ions
• The lattice structure is the responsible of the crystalline nature of ionic compounds

• The ions in the lattice are arranged in a three dimensional array with a regular repeating pattern
  • This systematic arrangement maximizes the Coulombic forces of attraction between positively charged ions and negatively charged ions
  • This arrangement also minimizes the Coulombic forces of repulsion between equally charged ions
• Ionic lattices tend to be close-packed and symmetric arrangements
• The overall charge of an ionic lattice is neutral

Ionic Lattice

 

 Ionic crystals are arranged in lattice structures

Properties of Ionic Crystals

• The physical properties of ionic crystals depend on the strength of the Coulombic attraction between the positive and negative ions
• The strength of the Coulombic forces increase if:
  • The charge of the ions increase because the force of attraction is proportional to the magnitude of the charge
    • g. The coulombic force of attraction is stronger in MgCl₂ than in NaCl
    • This occurs because Mg²⁺ has a 2+ charge and Na⁺ a 1+ charge
  • The ionic radius decrease because it is easier to attract an smaller ion rather than a bigger ion
    • g. The coulombic force of attraction is stronger in NaCl than in KCl
    • This occurs because Na⁺ has an smaller radius than K⁺

Table listing the characteristics of ionic crystals

Property	Explanation
Brittle	Ions can be splitted apart
High melting and boiling points	The Coulombic attraction between positive and negative ions in the lattice are strong
Generally soluble in water	The lattice is broken down and the ions can form ion-dipole interactions with the water molecules
Conduct electricity when in solution or molten	The lattice is broken down and there is  free movement of ions

 

Ionic Compound in a Solid and Molten / Solution

Molten or aqueous ions move freely but cannot in solid form