Johnsons Interstitial Electron Model for Metals

The description of metal bonding by interstitial electrons was given in 1972. In the interstitial electron model, atoms in metals shed some of their valence electrons, which occupy interstitial holes in the metal ion lattice. Not all of the valence electrons are in the interstices. The more electrons that are removed from the atoms, the more electronegative the cations become and the less they tend to donate further electrons to the interstices. Thus some valence electrons stay behind on the metal ions. How many depends on the atom. 

A bcc lattice has two octahedral interstices and three pseudotetrahedral interstices per atom. The ccp and hep lattices have one octahedral and two tetrahedral interstitial sites per atom. In metals the interstitial octahedral and tetrahedral sites between the positive metal ions are not all fully occupied some of them are only 

Copper has an fee lattice. The unit cell is described in the original model by its electronic structure 4Cu (d °), Icocu Half of the octahedral interstitial sites and a quarter of the tetrahedral sites are filled with free electrons. The copper lattice has Cu cations. 

Aluminum (also fee) has the electronic configuration 4AF , 2 gteti 4etet2, oct which means that, according to this model, there are two electron pairs in the first type of tetrahedral interstitial hole (tet ), four other electrons in each of the four other tetrahedral interstitial holes of tet2 of the cell, and four electrons in the four octahedral interstices. 

The bcc structures of iron, molybdenum, and tungsten are not close-packed as are those of fee and hep, and more interstitial sites are available for the free valence electrons. In the bcc lattice the octahedral holes are in the middle of the faces and in the center of the edges of the unit cell. Four tetrahedral sites form a circle in the eight faces of the unit cell. A second group of four tetrahedral sites forms a circle around all twelve edges of a cubic bcc unit cell. A maximum of two free electrons per face and per edge can be accommodated in the octahedral sites and two electrons 

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