Materials for engineering, 3rd Edition - (Malestrom)

6
Materials for engineering
materials consisting of compounds of metals with oxygen ions are largely
ionically bonded (MgO, Al 2 O 3 , ZrO 2 , etc).
Metallic bonding. About two-thirds of all elements are metals, and the
distinguishing feature of metal atoms is the looseness with which their valence
electrons are held. Metallic bonding is non-directional and the electrons are
more or less free to travel through the solid. The attractions between the
positive ions and the electron ‘gas’ give the structure its coherence, Fig. 1.3.
The limit to the number of atoms that can touch a particular atom is set by
the amount of room available and not by how many bonds are formed.
‘Close-packed’ structures, in which each atom is touched by twelve others,
are common and they give rise to the typical high density of metals. Since
each atom has a large number of neighbours, the overall cohesion is strong
and metals are therefore similar to ionic and covalent solids as regards
strength and melting point.
In general, the fewer the number of valence electrons an atom has and the
more loosely the electrons are held, the more metallic the bonding. Such
elements have high electrical and thermal conductivities because their valence
electrons are so mobile. Although a satisfactory description of some of the
physical properties of metals can be obtained from this ‘free electron’ picture,
many other properties (particularly those concerned with the motion of electrons
within metal crystals) have to be explained in terms of electrons as waves
occupying definite quantized energy states.
As the number of valence electrons and the tightness with which they are
held to the nucleus increase, they become more localized in space, increasing
the covalent nature of the bonding. Group IVB of the Periodic Table illustrates
particularly well this competition between covalent and metallic bonding:
diamond exhibits almost pure covalent bonding, silicon and germanium are
more metallic, tin exists in two modifications, one mostly covalent and the
other mostly metallic, and lead is mostly metallic.
Nucleus plus
inner electrons
Electron cloud
1.3 Classical model of a metal crystal.