Q.Why do metals have a large number of free electrons?
Step 1. In a metal atom, the electrons in the outermost shell are held to the nucleus by a relatively weak attractive force compared to inner-shell electrons.
Step 2. Because this binding is weak, thermal agitation at ordinary room temperature is already enough to free these outer electrons from their individual parent atoms.
Step 3. These freed electrons do not attach themselves to any one atom; instead they move randomly throughout the bulk of the metal, shared collectively by the whole lattice of positive ions -- this is exactly what is meant by 'free electrons' in a metal.
Step 4. Although these electrons move freely inside the metal, they still cannot leave the surface entirely, because the positive metal ions near the surface exert an attractive pull (the surface barrier) that must be separately overcome by electron emission.
Metals have a large number of free electrons because their outermost-shell electrons are loosely bound to the nucleus and become delocalised, moving randomly inside the metal even at room temperature.
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