Theories of metallic bonding

These are all changes which the theory of metal bonding would predict for the transition of metal - small cluster- -isolated atoms (94-97, 99-102, 136). However, before we ascribe the higher BE of small particles to bonding effects, we have to consider the following. 

The MO description of bonding in terms of filled MOs delocalised over all the atoms in the molecule reaches its fullest expression in the theory of metallic bonding, to which the rest of this chapter is devoted. 

A satisfactory theory of metallic bonding must account for the characteristic properties of high electrical and thermal conductivity, metallic lustre, ductility and the complex magnetic properties of metals which imply the presence of unpaired electrons. The theory should also rationalise the enthalpies of atomisation A/f tom of metallic elemental substances. A/f tom is a measure of the cohesive energy within the solid, and we saw in Chapter 5 how it plays an important part in the thermochemistry of ions in solids and solutions. The atomisation enthalpies of elemental substances (metallic and nonmetallic) are collected in Table 7.1. There is a fair correlation between A/Z tom an(J physical properties such as hardness and melting/boiling points. 

Let us now summarize some of the physical properties of metals in terms of the band theory of metallic bonding. 

The properties of metals are very different from those of ionic and covalent compounds. Many metals are strong and can he bent without breaking many are malleable (can be beaten into thin sheets) and ductile (can be drawn into wires). They are shiny when cut or polished and are excellent electrical and thermal conductors. Any theory of metallic bonding must account for all these physical properties and any differences or trends in these properties observed for metals. 

This theory of metallic bonding explains the physical properties of metals. If a stress is applied to the metal, the structure can change shape without the crystal fracturing. This is in contrast to the effect of stress on an ionic crystal, which will shatter. 

In the simplest theory of metallic bonding, the pool of electrons (blue) holds the metal together through interaction with the positive cores. 

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