Ionic Bonding & Structure (OCR A Level Chemistry A): Revision Note

Ionic bonding & structure

• Ionic bonding involves the transfer of electrons from a metal to a non-metal

  • Dot-and-cross diagrams can be used to help visualise the formation of positive and negative ions

• This forms ions with:

  • Full outer shells

  • Electronic configurations equivalent to a noble gas

Forming cations

• Metals lose electrons from their outer shell, forming positively charged cations

• For example:

  • Lithium loses its one outer electron to becomes a Li⁺ ion

  • The electronic configuration of Li⁺ is the same as He (1s²)

• Other examples:

  • Calcium loses its two outer electrons to become a Ca²⁺ ion

    • The electronic configuration of Ca²⁺ is the same as Ar (1s² 2s² 2p⁶ 3s² 3p⁶)

  • Aluminium loses its three outer electrons to become an Al³⁺ ion

    • The electronic configuration of Al³⁺ is the same as Ne (1s² 2s² 2p⁶)

Forming anions

• Non-metals gain electrons into their outer shell, forming negatively charged anions

• For example:

  • Oxygen gains two electrons

  • It becomes an O^2- ion

  • The electronic configuration of O^2- is the same as Ne (1s² 2s² 2p⁶)

Other examples:

• Chlorine gains one electron to become a Cl^- ion

  • The electronic configuration of Cl^- is the same as Ar (1s² 2s² 2p⁶ 3s² 3p⁶)

• Nitrogen gains three electrons to become an N^3- ion

  • The electronic configuration of N^3- is the same as Ne (1s² 2s² 2p⁶)

Forming ionic bonds

• Ionic bonds are formed when electrons are transferred from one species to another, forming cations and anions

• For example, NaCl:

• Cations and anions are oppositely charged

• These oppositely charged ions are held together by strong electrostatic attractions, forming an ionic bond

• An ionic bond is the electrostatic attraction formed between the oppositely charged ions

  • This occurs in all directions and is 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

Giant ionic lattices

• Ionic compounds form giant ionic lattices

  • They are 3D networks of oppositely charged ions

• Giant ionic lattices are held together by strong electrostatic attractions between ions in all directions (non-directional bonding)

• The lattice is a regular, repeating pattern of alternating cations and anions

• The shape and arrangement of the lattice depends on the relative sizes of the ions

  • Many common lattices, like NaCl and MgO, are cubic in shape

Physical properties of ionic compounds

• 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

• The strong electrostatic forces of attraction between oppositely charged ions require large amounts of energy to overcome

• The strength of ionic bonding increases with ionic charge

  • More highly charged ions create stronger attractions

• 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

• Solubility depends on the balance between ionic bonding and the attraction between ions and polar solvent molecules

• 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

    • 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

  • So, the ions are free to move and carry charge