Conductor band, electron movement

Conductors (metals). The valence and conduction bands of a conductor have no gap between them, so electrons flow when even a tiny electrical potential difference is applied. When the temperature is raised, greater random motion of the atoms hinders electron movement, which decreases the conductivity of a metal. 

In a metal, certainly the transition metals, the electrons are more or less free to move in conduction bands. This fact is responsible for the high electrical conductivity of metals. When hydrogen atoms are present in the holes between the atoms, the movement of the electrons is somewhat impaired. As a result, the metal hydrides of this class are poorer conductors than the pure metals. The presence of hydrogen atoms makes the metal atoms less mobile and more restricted to particular lattice sites. Accordingly, the interstitial metal hydrides are more brittle than the parent metal. Also, the inclusion of the hydrogen atoms causes a small degree of lattice expansion so that the interstitial hydrides are less dense than the parent metal alone. 

On the contrary, the VB is completely filled in the case of ionic or covalent solids but it is separated by a high energy gap from the subsequent empty band. In this situation no electrons can move even if high electric fields are applied and the solid is an insulator. If the forbidden energy gap is not so high, some electrons could pass in the energetic empty band by means of thermal excitation, and the material behaves as a weak conductor, that is, as a semiconductor. The empty band, which allows the movement of the electrons, is called conduction band (CB). 

Because the valence band and the conduction band are adjacent to each other, the amount of energy needed to promote a valence electron to the conduction band is negligible. There the electron can travel freely through the metal because the conduction band is void of electrons. This freedom of movement accounts for the fact that metals are good conductors, that is, they are capable of conducting electric current. 

Metals have no band gap, so they are good conductors of electricity. The valence band and conduction band actually overlap or are one and the same in metallic conductors, so there is no energy barrier to the movement of electrons from one atom to another in a metal. Semiconductors have a band gap, but it is relatively small, so there is limited movement of electrons from the valence band to the conduction band. Nonconductors (electrical insulators) have large band gaps, so it is nearly impossible to promote electrons from the valence band to the conduction band. 

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