1.6.5 Metallic Bonding & Lattices

• Metal atoms are tightly packed together in lattice structures
• When the metal atoms are in lattice structures, the electrons in their outer shells are free to move throughout the structure
• The free-moving electrons are called ‘delocalised electrons’ and they are not bound to their atom
• When the electrons are delocalised, the metal atoms become positively charged ions
• The positive charges repel each other and keep the neatly arranged lattice in place
• There are very strong forces between the positive metal centres and the ‘sea’ of delocalised electrons

The positive metal ions are suspended in a ‘sea’ of delocalised electrons

• 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
• This layered structure with the delocalised electrons gives a metal its key properties 

Layers of copper ions (the delocalised electrons are not shown in the diagram)

 

• If other atoms are added to the metal structure, such as carbon atoms, this creates an alloy
• Alloys are much stronger than pure metals, because the other atoms stop the layers of metal ions sliding over each other easily

• The strength of the metallic attraction can be increased by:
  • Increasing the number of delocalised electrons per metal atom
  • Increasing the positive charges on the metal centres in the lattice
  • Decreasing the size of the metal ions

• Due to the delocalised ‘sea’ of electrons, metallic structures have some characteristic properties shown below:

Metallic Bonding Properties Table 

Property	Reason
High melting and boiling point	There are strong electrostatic forces of attraction between the delocalised electrons and positive metal ions which results in strong metallic bonds within giant metallic structures. These require large amounts of energy to overcome the forces and break the bonds
Good conductors of electricity	The delocalised electrons are able to move and carry charge. For example, in a circuit, electrons entering one end of the metal cause a delocalised electron to be displaced from the other need. Hence, electrons can flow and conduct electricity
Good conductors of heat	The delocalised electrons are free to move within the metal structure. The conversion of the kinetic energy of the electron to heat energy when it collides with the metal atoms/nuclei results in the transfer of energy. This is alongside the increased vibrations and movement of the particles (positive ions and delocalised electrons)
Malleable and ductile	Layers of positive ions can easily slide over one another to take up different positions. This does not disrupt the metallic bonding as the delocalised electrons do not belong to any particular metal atom and so they can move with the layers of positive ions, maintaining the electrostatic forces. Therefore, the metallic bonds are not broken and, as a result, metallic bonds are strong but flexible/