Throughout my highschool classes, I have been made to learn that metals have free electrons that's why they are able to conduct electricity.. But I never understood why. Is that related to metallic bonding… Correct me if I am wrong but even if that's the case…. I am just not able to understand the concept of free electrons
Free Electrons in Metals – Why Do Metals Have Free Electrons
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Best Answer
Without getting into the quantum mechanical details, here’s a cartoon depiction of what’s going on. The vertical axis represents energy.
Like other answers have already pointed out, metals don’t have actual free electrons. In the cartoon this is given by the grey region. If electrons have enough energy to be in the grey region, they’re free.
In individual independent atoms (gaseous state), the energy levels below a certain energy are discrete. This is depicted by the lines in the cartoon. This means the energy is fixed, rigid. The electrons in this state can’t conduct electricity.
In solids however, the discrete states of multiple neighbouring atoms “merge” into a continuum and create what is called as bands. For further details you may look at my answers here.
With this, there exist a continuum of states called the conduction band where the electrons are not bound to any single atom of the solid. They are mobile. The fascinating property of these states is that it is possible for electrons to respond to an external electric field. These states are called Bloch waves.
In insulators there is a big energy gap between the filled states (valence) and the empty states (conduction). So without sufficient external field, they are unable to conduct electricity.
In metals however, the energy gap is absent and thus electrons can easily go into the conduction band and respond to external electric field.
Some details
The reason why mobile electrons seem like free electrons has to do with crystal symmetries. Specifically translational symmetry. In a crystal the atoms are arranged in a regular periodic manner. In the bulk (non boundary) of the metal if you go from one atom to another, the neighbourhood looks identical. This is known as translational symmetry. And a consequence of this is that the electrons have well defined momentum, just as a free electron does. This is encapsulated in the band structure.