Metallic bonding
Metallic bonding occurs between the atoms of metal elements - Lithium, Beryllium, Sodium, Magnesium, Aluminium and Calcium. The outer electrons are Electrons that are not associated with a particular atom, eg in a metal, outer electrons can be free to move through the solid. (free to move).
This produces an electrostatic force of attraction between the positive metal ions and the negative delocalised electrons.
This delocalised 'sea of electrons' is responsible for metal elements being able to conduct electricity.