The Basics of Conductors and Insulators
When it comes to solids, we can generally categorize them into three groups: conductors, insulators, and semiconductors. The defining characteristic of these groups is their ability, or lack thereof, to allow electric charge to flow through them - a trait known as conductivity. This conductivity can vary greatly among solids, from 10-20 to 107 ohm-1 m-1.
Solids with a conductivity that falls between 104 and 107 ohm-1 m-1 are deemed conductors. Many metals, for example, have a conductivity of 107 ohm-1 m-1, making them excellent conductors of electricity.
The atomic structure of a material plays a significant role in its conductivity. In conductive materials like metals, the outermost electrons are not tightly bound and can move freely, facilitating the flow of electric charge. However, in insulating materials, the electrons are closely bound, inhibiting their movement and thus, the flow of electricity. An example of an insulator is rubber.
The band theory provides an explanation for the conductivity of solids. Two bands, the conduction band and the valence band, exist in a solid. The valence band contains the highest energy electrons, while the conduction band is the range with the lowest vacant electronic state. The valence band is below the Fermi level, and the conduction band is above it.
Conductors are characterized by the overlapping of the valence and conduction bands, leading to a negligible gap between them. This overlap allows for a high degree of electron transfer, giving these materials high conductivity. In contrast, insulators have a large gap between the two bands, preventing electron transfer and thus, making them poor conductors of electricity.