What is the reason for the electrical conductivity of metals?

In metals, atoms are so tightly packed that the electron of one atom experiences a rather significant force of other closed atoms. As a result, the valence and conduction bands in metals approach each other and may even overlap. Hence, receiving very little energy from external heat or a source of electrical energy, electrons can easily rise to higher levels in the metal. Such electrons are known as free electrons. These free electrons are responsible for the current flowing through the metal. When an external electrical source is connected to a piece of metal, these free electrons begin to flow to the higher potential output of the source, causing current to flow in the metal. In a metal, the density of free electrons in the conduction band is much higher than that of other materials, which is why the metal is called a very good electrical conductor.