<P> The delocalization is most pronounced for s - and p - electrons . For caesium it is so strong that the electrons are virtually free from the caesium atoms to form a gas constrained only by the surface of the metal . For caesium, therefore, the picture of Cs ions held together by a negatively charged electron gas is not too inaccurate . For other elements the electrons are less free, in that they still experience the potential of the metal atoms, sometimes quite strongly . They require a more intricate quantum mechanical treatment (e.g., tight binding) in which the atoms are viewed as neutral, much like the carbon atoms in benzene . For d - and especially f - electrons the delocalization is not strong at all and this explains why these electrons are able to continue behaving as unpaired electrons that retain their spin, adding interesting magnetic properties to these metals . </P> <P> Metal atoms contain few electrons in their valence shells relative to their periods or energy levels . They are electron deficient elements and the communal sharing does not change that . There remain far more available energy states than there are shared electrons . Both requirements for conductivity are therefore fulfilled: strong delocalization and partly filled energy bands . Such electrons can therefore easily change from one energy state into a slightly different one . Thus, not only do they become delocalized, forming a sea of electrons permeating the lattice, but they are also able to migrate through the lattice when an external electrical field is imposed, leading to electrical conductivity . Without the field, there are electrons moving equally in all directions . Under the field, some will adjust their state slightly, adopting a different wave vector . As a consequence, there will be more moving one way than the other and a net current will result . </P> <P> The freedom of conduction electrons to migrate also give metal atoms, or layers of them, the capacity to slide past each other . Locally, bonds can easily be broken and replaced by new ones after the deformation . This process does not affect the communal metallic bonding very much . This gives rise to metals' typical characteristic phenomena of malleability and ductility . This is particularly true for pure elements . In the presence of dissolved impurities, the defects in the lattice that function as cleavage points may get blocked and the material becomes harder . Gold, for example, is very soft in pure form (24 - karat), which is why alloys of 18 - karat or lower are preferred in jewelry . </P> <P> Metals are typically also good conductors of heat, but the conduction electrons only contribute partly to this phenomenon . Collective (i.e., delocalized) vibrations of the atoms known as phonons that travel through the solid as a wave, contribute strongly . </P>

How does metallic bonding account for the properties of metals