• To explain the properties of metals like iron or sodium we need a more sophisticated picture than a simple particle model of atoms all lined up in close packed rows and layers.
• A giant metallic lattice. The crystal lattice of metals consists of ions (NOT atoms) surrounded by a 'sea of electrons' forming another type of giant lattice.
• The outer electrons (-) from the original metal atoms are free to move around between the positive metal ions formed (+).
• These free or 'delocalised' electrons are the 'electronic glue' holding the particles together.
• There is a strong electrical force of attraction between these free and mobile electrons (-) and the 'immobile' positive metal ions (+) and this is the metallic bond.
• Metallic bonding is not directional like covalent bonding, it is like ionic bonding in the sense that the force of attraction between the positive metal ions and the mobile electrons acts in every direction about the fixed (immobile) metal ions.

• Metals can become weakened when repeatedly stressed and strained. This can lead to faults developing in the metal structure called 'metal fatigue' or 'stress fractures'. If the metal fatigue is significant it can lead to the collapse of a metal structure. So it is important develop alloys which are well designed, well tested and will last the expected lifetime of the structure whether it be part of an aircraft (eg titanium aircraft frame) or a part of a bridge (eg steel suspension cables).

 Note on Alloy Structure

1. Shows the regular arrangement of the atoms in a pure metal crystal and the white spaces show where the free electrons are (yellow circles actually positive metal ions).
2. Shows what happens when the metal is stressed by a strong force. The layers of atoms can slide over each other and the bonding is maintained as the mobile electrons keep in contact with atoms, so the metal remains intact BUT a different shape.
3. Shows an alloy mixture. It is NOT a compound but a physical mixing of a metal plus at least one other material (shown by red circle), it can be another metal e.g. Ni, a non-metal e.g. C or a compound of carbon or manganese, and it can be bigger or smaller than iron atoms. Many alloys are produced to give a stronger metal. The presence of the other atoms (smaller or bigger) disrupts the symmetry of the layers and reduces the 'slip ability' of one layer next to another. The result is a stronger harder less malleable metal.
4. The main point about using alloys is that you can make up, and try out, all sorts of different compositions until you find the one that best suits the required purpose in terms of tensile/compression strength, malleability, electrical conductivity or corrosion resistance etc.