Inorganic Chemistry
359 pág.

Inorganic Chemistry

DisciplinaQuímica Inorgânica I3.906 materiais32.034 seguidores
Pré-visualização50 páginas
net motion of electric charge. For conduction to occur in a nonmetallic solid, 
therefore, some electrons must be excited from the VB to the CB. This gives rise to an activation 
energy, and conductivity increases 
Key Notes 
The band 
Metallic solids have a continuous band of electronic energy levels with the top filled level, the 
Fermi level, within it. In nonmetallic solids there is a bandgap separating the filled valence band 
from the empty conduction band. 
Bandgaps Bandgaps determine the optical absorption of a nonmetallic solid and the possibility of 
semiconduction. Bandgaps in binary solids decrease with decreasing electronegativity 
difference between the elements. In most ionic and covalent solids bandgaps are smaller with 
elements in lower periods. 
The static dielectric constant of a solid arises from the displacement of ions in an electric field 
and may be particularly large for some ionic solids. The high-frequency dielectric constant 
depends on electronic polarizability and determines the optical refractive index. 
Influence of 
Defects including impurities have a major influence on the electrical properties of nonmetallic 
solids. They can provide extra electrons or holes, which enhance semiconduction, and they can 
also facilitate conduction by ions. 
Related topicElement structures (D2) 
Página 1 de 1
Page 121
Fig. 1. Band picture for (a) nonmetallic and (b) metallic solid; occupied electronic levels 
are shown shaded. 
with rise in temperature approximately in accordance with the Arrhenius equation used in chemical 
kinetics (see Topic B3). 
Nonmetallic solids include ionic and covalent compounds. In the former case, the VB is made up 
of the top filled anion levels (e.g. the 3p orbitals of Cl\u2212, which are filled in making the ion) and the 
CB of the lowest empty cation levels (e.g. in Na+ the 3s level from which an electron has been 
removed to make the cation). In covalent solids such as diamond the VB consists of bonding orbitals 
(e.g. C\u2014C) and the CB of antibonding orbitals. 
Simple metallic solids are elements or alloys with close-packed structures where the large number 
of interatomic overlaps gives rise to wide bands with no gaps between levels from different atomic 
orbitals. Metallic properties can arise, however, in other contexts. In transition metal compounds a 
partially occupied d shell can give rise to a partly filled band. Thus rhenium in ReO3 has the formal 
electron configuration 5d1 (see Topic H1) and is metallic. WO3 (formally 5d
0) is not metallic but 
Na0.7WO3 is, as electrons from sodium occupy the band made up of W 5d orbitals (see Topic D5). 
The bandgap in a nonmetallic solid is important for electrical and optical properties. A solid with a 
small bandgap is a semiconductor with a conductivity that (unlike the case with a metal) increases 
as temperature is raised. The bandgap also determines the minimum photon energy required to excite 
an electron from the VB to the CB, and hence the threshold for optical absorption by a solid. 
In a covalent solid the bandgap is related to the energy splitting between bonding and antibonding 
orbitals (see Topic C4) and thus to the strength of bonding. In an ionic solid the bandgap is 
determined by the energy required to transfer an electron back from the anion to cation, which is 
related to the lattice energy (see Topic D6). Bandgaps for elements and binary compounds follow 
some systematic trends. 
A comparison between compounds of pre-transition metals (e.g. Ca) and corresponding post-
transition metals (e.g. Cd) provides a good example of the influence 
\u2022 In a series of isoelectronic solids such as CuBr-ZnSe-GaAs-Ge the bandgap decreases with 
decreasing electronegativity difference between the two elements. This trend reflects the 
decreasing energy difference between \u2018anion\u2019 and \u2018cation\u2019 orbitals. 
\u2022 In series such as C-Si-Ge or LiF-NaF-KF the bandgap decreases as the group is descended and 
atoms or ions become larger. This trend reflects the decline in bond or lattice energies with 
larger atoms or ions (see Topics C8 and D6). 
Página 1 de 1
Page 122
of the electronegativity differences (see Topic G1). Bandgaps are smaller in compounds of the less 
electropositive post-transition metals. The colors of CdS and CdSe (used as yellow and red 
pigments) come from strong absorption of blue light, as the bandgaps correspond to photon energies 
in the visible spectrum. Analogous calcium compounds are not colored as the larger bandgaps 
correspond to UV radiation. 
Dielectric properties 
The dielectric constant of a medium is a measure of the electrostatic polarization, which reduces the 
forces between charges (see Topics C10 and E1 for liquids). Two different mechanisms contribute to 
the dielectric properties of a solid according to the time-scale involved. The static dielectric 
constant depends on the displacement of ions from their regular positions in an applied electric field. 
It is applicable for static fields, or frequencies of electromagnetic radiation up into the microwave 
range. The high-frequency dielectric constant is measured at frequencies faster than the vibrational 
motion of ions. It is applicable in the visible region of the spectrum, and determines the refractive 
index, which governs the transmission of light in transparent media. 
As expected, ionic substances have higher static dielectric constants than nonionic ones. 
Especially large values arise when ions can be easily displaced from their positions in the regular 
structure. For example, barium titanate BaTiO3 has a very high dielectric constant that varies with 
temperature. In the perovksite structure (see Topic D5) the large Ba2+ ion imposes a relatively large 
O\u2014O distance so that Ti4+ can move easily out of the center of its octahedral site. Below 120°C a 
permanent distortion sets in, which gives each unit cell a dipole moment. This type of behavior is 
called ferroelectric and has important applications, for example, in capacitors for electronic circuits. 
Large high-frequency dielectric constants (and hence refractive indices) depend not on ionic 
motion but on electronic polarizability. Large ions contribute to this, and glasses containing Pb2+ are 
traditionally used for lenses where a high refractive index is necessary. Electronic polarizability can 
also be large in compounds with small bandgaps. A gap outside the visible spectrum is necessary for 
a colorless material in optical applications. TiO2 is used as a white pigment because it has the right 
optical properties combined with cheapness, chemical stability and non-toxicity. The bandgap is only 
just in the UV, and the refractive index in the visible spectrum is high. Each grain is highly 
reflective, and a powdered sample appears white because light is reflected in random directions. 
Influence of defects 
All solids contain defects where the regularity of the ideal periodic lattice is broken. Line and plane 
defects (dislocations, grain boundaries, etc.) are important for mechanical properties but it is point 
defects that are most significant for electrical properties. They include 
Defects that introduce extra electrons, or that give missing electrons or \u2018holes\u2019, have a large 
influence on electronic conduction in nonmetallic solids. Most semiconductor devices use doped or 
extrinsic semiconductors rather than the intrinsic semiconduction of the pure material. Doping Si 
with P replaces some tetrahedrally bonded Si atoms in the diamond lattice (see Topic D2) with P. 
Each replacement provides one extra valence electron,