Metallic Bonding Where does this type of bonding go on the continuum of bonding? ©2011 University of Illinois Board of Trustees

Models of metallic bonding Electron-sea model Molecular orbital model ©2011 University of Illinois Board of Trustees

Electron-sea model ©2011 University of Illinois Board of Trustees

Electron-sea model Simplest picture of metals, Regards them as a lattice of positive ions immersed in a “sea of electrons” Electrons can freely migrate throughout the solid Electrons experience electrostatic forces of attraction to the nuclei and repulsion to other electrons ©2011 University of Illinois Board of Trustees

Properties of metals explained by this model The electropositive nature of the metallic atoms allows their valence electrons to exist as a mobile fluid which can be displaced by an applied electric field, hence giving rise to their high electrical conductivities. ©2011 University of Illinois Board of Trustees

Additional properties of metals Because each ion is surrounded by the electron fluid in all directions, the bonding has no directional properties; this accounts for the high malleability and ductility of metals. ©2011 University of Illinois Board of Trustees

Properties of metals (cont) Metallic luster refers to the ability of reflect light. When light falls on a metal, its rapidly changing electromagnetic field induces similar motions in the more loosely-bound electrons near the surface which are not bound to specific atoms A vibrating charge is itself an emitter of electromagnetic radiation, so the effect is to cause the metal to re-emit, or reflect, the incident light, producing the shiny appearance. ©2011 University of Illinois Board of Trustees

Molecular-orbital model Red valence electrons on the right are now delocalized as the pink area on the left. ©2011 University of Illinois Board of Trustees

Molecular orbital model The electrons can move freely within these molecular orbitals, and so each electron becomes detached from its parent atom. The electrons are said to be delocalized. The metal is held together by the strong forces of attraction between the positive nuclei and the delocalized electrons. ©2011 University of Illinois Board of Trustees

Examples of MO theory - Na Sodium has the electronic structure 1s 2 2s 2 2p 6 3s 1. The electron in the 3s atomic orbital of one sodium atom shares space with the corresponding electron on a neighboring atom to form a molecular orbital Each sodium atom is being touched by eight other sodium atoms - and the sharing occurs between the central atom and the 3s orbitals on all of the eight other atoms. All of the 3s orbitals on all of the atoms overlap to give a vast number of molecular orbitals which extend over the whole piece of metal. There have to be huge numbers of molecular orbitals, of course, because any orbital can only hold two electrons. ©2011 University of Illinois Board of Trustees

MO model using Li ©2011 University of Illinois Board of Trustees

Electrostatic potential diagram from WebMO for Li 4 ©2011 University of Illinois Board of Trustees

Properties explained by MO model Strength of metallic bonding first increases with number of electrons and then decreases. This model can explain the decrease in bond strength as the larger number of valence electrons results in more antibonding orbitalsMelting points vs number of electrons in metals ©2011 University of Illinois Board of Trustees

Melting points of metals vs number of electrons in metals ©2011 University of Illinois Board of Trustees

Band Model/Theory ©2011 University of Illinois Board of Trustees

Band Model In most metals there will be bands derived from the outermost s-, p-, and d atomic levels, leading to a system of bands, some of which will overlap as described above. Where overlap does not occur, the almost continuous energy levels of the bands are separated by a forbidden zone, or band gap. Only the outermost atomic orbitals form bands; the inner orbitals remain localized on the individual atoms and are not involved in bonding ©2011 University of Illinois Board of Trustees

Properties due to Band Model What color is a metal? With the two exceptions of copper and gold, the closely-spaced levels in the bands allow metals to absorb all wavelengths equally well, so most metals are basically black, but this is ordinarily evident only when the metallic particles are so small that the band structure is not established. ©2011 University of Illinois Board of Trustees

What causes the color of gold? The distinctive color of gold is a consequence of Einstein's theory of special relativity acting on the extremely high momentum of the inner- shell electrons, increasing their mass and causing the orbitals to contract. The outer (5d) electrons are less affected, and this gives rise to increased blue-light absorption, resulting in enhanced reflection of yellow and red light. ©2011 University of Illinois Board of Trustees

Sources #top #top html#SEC2 html#SEC2 Brown, LeMay, Bursten. Chemistry: The Central Science. 11 th Edition. Pearson-Prentice Hall. Saddle Brook, NJ ©2011 University of Illinois Board of Trustees