Chemistry: The Molecular Nature of Matter and Change 7th Edition

Published by McGraw-Hill Education
ISBN 10: 007351117X
ISBN 13: 978-0-07351-117-7

Chapter 12 - Problems: 12.103

Answer

See explanation below.

Work Step by Step

1 Concept : Like metal atoms, large numbers of nonmetal and metalloid atoms can form bands of Molecular Orbitals. Band theory explains differences in electrical conductivity and the effect of temperature among these three classes of substances in terms of the presence of an energy gap between their valence and conduction bands. Following are the three types of elements on this basis : Conductors : The valence and conduction bands of a conductor have no energy gap between them, so electrons flow when a tiny electrical potential difference is applied. When the temperature is raised, greater random motion of the atoms hinders electron movement: conductivity decreases when a metal is heated. Semiconductors : In a semiconductor, a small energy gap exists between the valence and conduction bands. Thermally excited electrons can cross the gap, allowing a small current to flow: in contrast to a conductor, conductivity increases when a semiconductor is heated. Insulators : In an insulator, a large energy gap exists between the bands: no current is observed even when the substance is heated. 2 (a) Silicon : Silicon is a Metalloid, and hence can be classified as a semiconductor. The conductivity of semiconductors increases on heating, as discussed above. Hence, when Silicon is heated, it's conductivity will increase and consequently, conductivity will decrease with decrease in Temperature. Ans :Conductivity Decreases 3 (b) Lead : Lead is a metal, and hence is classified as a conductor. When the temperature is raised, greater random motion of the atoms hinders electron movement in metals, and thus, conductivity decreases when a metal is heated and consequently, conductivity will increase with decrease in Temperature. Ans : Conductivity Increases 4 (c) Germanium : Germanium is a Metalloid, and hence can be classified as a semiconductor. The conductivity of semiconductors increases on heating, as discussed above. Hence, when Germanium is heated, it's conductivity will increase and consequently, conductivity will decrease with decrease in Temperature. Ans : Conductivity Decreases
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