2.4.2 Ionic Bonding & Structure

• As a general rule, metals are on the left of the periodic table and nonmetals are on the right-hand side
• Ionic bonding involves the transfer of electrons from a metallic element to a non-metallic element
• Transferring electrons usually leaves the metal and the non-metal with a full outer shell
• Metals lose electrons from their valence shell forming positively charged cations
• Non-metal atoms gain electrons forming negatively charged anions
• Once the atoms become ions, their electronic configurations are the same as a noble gas
  • A potassium ion (K⁺) has the same electronic configuration as argon: [2,8,8]⁺
  • A chloride ion (Cl^-) also has the same electronic configuration as argon: [2,8,8]^-

Forming cations by the removal of electrons from metals

Forming anions by the addition of electrons to nonmetals

• Cations and anions are oppositely charged and therefore attracted to each other
• Electrostatic attractions are formed between the oppositely charged ions to form ionic compounds
• The ionic bond is the electrostatic attraction formed between the oppositely charged ions, which occurs in all directions ( this called non-directional bonding)
• This form of attraction is very strong and requires a lot of energy to overcome
  • This causes high melting points in ionic compounds

 

Cations and anions bond together using strong electrostatic forces, which require a lot of energy to overcome

• 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

 

General ionic lattice which shows the actual packing of the ions

• The giant ionic lattice and ionic bonding can be used to explain many of the physical properties of ionic compounds

Melting and boiling point

• Most ionic compounds are solids at room temperature
• This is because there isn't enough energy to overcome the strong electrostatic forces of attraction between the oppositely charged ions that make up the lattice
• Therefore, high temperatures are required to make an ionic compound melt or boil
• Melting (and boiling) points are also higher for lattices that contain ions with a greater ionic charge
  • For example, the melting point of sodium oxide, Na₂O, is 1405 K while the melting point of calcium oxide, CaO, is 2845 K
  • This is due to a stronger attraction between the ions - the size of the ions is not a factor here as the Na⁺ and Ca²⁺ ions are a similar size

Solubility

• Many ionic compounds will dissolve in polar solvents, e.g. water
• Solubility is dependent on two main factors:
  1. Breaking down the ionic lattice
  2. The polar molecules attracting and surrounding the ions
• Polar molecules, such as water, can break down or disrupt the ionic lattice and surround each ion in solution
• The δ+ end of the polar molecule can surround the negative anion
• The δ- end of the polar molecule can surround the positive cation
• The solubility of an ionic compound depends on the relative strength of the electrostatic forces of attraction within the ionic lattice and the attractions between the ions and the polar molecule
• In general, the greater the ionic charge the less soluble an ionic compound is
  • For example, 356.9 g of sodium chloride, NaCl, will dissolve in one dm³ of water while only 74.4 g of calcium chloride will dissolve in one dm³ of water 
  • This is a general rule though and there are many exceptions

Electrical conductivity

• Ionic compounds do not conduct electricity when solid
  • This is because the ions are in fixed positions within the solid lattice so there are no mobile charge carriers 
• Ionic compounds can conduct electricity when they are molten or aqueous
  • This is because the ions are no longer in fixed positions as the lattice has broken down, therefore, the ions are free to act as mobile charge carriers