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  4. Pure sodium (Na) is a good conductor of electricity because the 3s and 3p atomic bands overlap to form a partially filled conduction band. By contrast the ionic sodium chloride (NaCl) crystal is

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Q. Pure sodium $(Na)$ is a good conductor of electricity because the $3s$ and $3p$ atomic bands overlap to form a partially filled conduction band. By contrast the ionic sodium chloride ($NaCl$) crystal is

Semiconductor Electronics: Materials Devices and Simple Circuits

Solution:

In sodium chloride the $Na ^{+}$ and $Cl ^{-}$ ions both have noble gas electron configuration corresponding to completely filled bands. Since the bands do not overlap, there must be a gap between the filled bands and the empty bands above them, so $NaCl$ is an insulator.