How do metals conduct electrons/electricity?

Their electrons are free and can move under force. This is the simplest explanation. An applied votlage can create an electric field that causes this force. A current is the movement of electrons.

However, this doesn’t tell us much because we don’t understand why they are free. We need to look at band theory for that.

An electron can only take on discrete energies in a single atom. This is the case for the 2s and 2p orbitals shown below. Molecular orbitals are formed when atoms combine their orbitals to form molecules. Keep in mind that the number and size of the orbitals remains the same. A bonding orbital can also be formed. However, an antibonding orbital may also be formed.

The number of molecular orbitals will increase by adding atoms and molecules to our molecule. However, the range of energies does not grow by as much. As the molecule grows, energy states become closer and more similar in energy.

The states of a crystal solid, which is basically an infinite number of molecules, are so close that they effectively create a continuous spectrum. Because of the small difference in energy, electrons can jump from one state to the next without difficulty. They can move from one state to the next because they can jump between them.

Hence, why can’t all solids conduct electric current?

An electron must be able to jump to another state in order to be able. Pauli exclusion principle states that two electrons cannot occupy the same state. Metals can have either half-filled or overlapping bands which means that there are many states available for electrons to jump into.

The highest occupied band of insulators is fully filled. This means that an electron must hop into the next band to find an empty state. This takes considerable energy. The band gap is the energy that electrons cannot overcome. This means that there is no charge movement or hopping.

Materials can form intermediate cases where there is a bandgap but only a small fraction of electrons can overcome it. These materials conduct electricity, but they are limited in the number of electrons available to them.

Images taken from google images.

This explanation is still qualitative, in that it doesn’t explain why metals can have overlapping or half-filled bands. It doesn’t explain why certain metals conduct better than others. This is due more to scattering mechanisms that to the number or free states of electrons.


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