Insulators
المؤلف:
Peter Atkins, Tina Overton, Jonathan Rourke, Mark Weller, and Fraser Armstrong
المصدر:
Shriver and Atkins Inorganic Chemistry ,5th E
الجزء والصفحة:
104
2025-08-24
396
Insulators
Key point: A solid insulator is a semiconductor with a large band gap. A solid is an insulator if enough electrons are present to fill a band completely and there is a considerable energy gap before an empty orbital becomes available (Fig. 3.71). In a sodium chloride crystal, for instance, the NCl- ions are nearly in contact and their 3s and three 3p valence orbitals overlap to form a narrow band consisting of 4N levels. The Na ions are also nearly in contact and also form a band. The electronegativity of chlorine is so much greater than that of sodium that the chlorine band lies well below the sodium band, and the band gap is about 7 eV. A total of 8N electrons are to be accommodated (seven from each Cl atom, one from each Na atom). These 8N electrons enter the lower chlorine band, fill it, and leave the sodium band empty. Because the energy of thermal motion available at room temperature is kT ≈ 0.03 eV (k is Boltzmann’s constant), very few electrons have enough energy to occupy the orbitals of the sodium band.
In an insulator the band of highest energy that contains electrons (at T=0) is normally termed the valence band. There next higher band (which is empty at T=0) is called the conduction band. In NaCl the band derived from the Cl orbitals is the valence band and the band derived from the Na orbitals is the conduction band.
We normally think of an ionic or molecular solid as consisting of discrete ions or mol ecules. According to the picture just described, however, they can be regarded as having a band structure. The two pictures can be reconciled because it is possible to show that a full band is equivalent to a sum of localized electron densities. In sodium chloride, for example, a full band built from Cl orbitals is equivalent to a collection of discrete Cl- ions.
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