Ionic compounds

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Conductors. insulators and semiconductors

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  In electrical conductors (metals), either the valence band is oniy partly fu1l, or the valence and conductiori bands overlap. There is therefore no significant gap between filled and un_filled MOs, and perturbation can occur readily. In insulators (non-metals), the valence band is full, so perturbation within the band is impossible, and there is an appreciable difference in energy (called the band gap) between the valence band and the next empty band. Electrons cannot therefore be promoted to an empty level where they could move freely. Intrinsic semiconductors are basically insulators, where the energy gap between adjacent bands is sufficiently small for thermal energy to be able to promote a small number of electrons from the full valence band to the empty conduction band. Both the promoted electron in the conduction band and the unpaired electron left in the valence band can conduct electricity. The conductivity of semiconductors increases with temperature, because the number ...
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Electron affinity

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  The energy released when an extra. electron is added to a neutrai gaseous atom is termed the electron affinity. Usually only one electron is added, forming a uninegative ion. This ~epels further electrons and energy is needed to add on a second electron: hence· the negative affinity of 0 2-. Electron affinities depend on the size and effective nuclear charge. They cannot be determined directly, but are obtained indirectly from the Born-Haber cycle.  Negative Clectron affinity values indicate that energy is.given out when the atom accepts an electron. The above values show that the halogens all evolve a large amount of energy on forming negative halide ions, and it is not surprising that these ions occur in. a large number of compounds. Energy is evolved when one electron is added to an 0 or S atom, forming the species o- and s-, but a substantial amount of energy is absorbed when two electrons are added to form 0 2- and s2- ions. Even though it requires energy to form these ...
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Crystal structures of metals

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  Metallic elements usually have a close-packed structure with a coordination number of 12. There are two types of close packing depending on the arrangement of adjacent layers in the structure: cubic close packing ABCABC and hexagonal close packing ABAB (see Metallic bonds and metallic structures in Chapter 2). However, some metals have a body centred cubic type of structure (which fills the space slightly less efficiently) where there are eight nearest neighbours, with another six next-nearest neighbours about 15% further away. If this small difference in distance between nearest and next-nearest neighbours is disregarded, the coor-dination number for a body-centred cubic structure may be regarded loosely as 14. The mechanical properties of malleability and ductility depend on the ease with which adj~nt planes of atoms can glide over each other, to give an equivalent arrangement of spheres. These properties are also affected by physical imperfections such as grain boundaries and ...
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