Metals and Solid-State Materials
Interactive Student Tutorial

 21.4 Bonding in Metals

To explain the characteristic properties of metals (ductility, malleability, high thermal and electrical conductivity), two different models are used.

 Electron-Sea Model of Metals

In the electron-sea model, a metal crystal is considered to be a three-dimensional array of metal cations immersed in a sea of valence electrons. The delocalized valence electrons are free to move throughout the crystal and are not associated with any one particular metal cation. The mobility of the electrons accounts for the high electrical conductivity of metals. Thermal conductivity can also be ascribed to the mobile electrons that conduct heat by carrying kinetic energy from one part of the crystal to another. Three-dimensional delocalized bonding allows the metal to be both malleable and ductile.

 Molecular Orbital Theory for Metals

Molecular orbital (MO) theory for small molecules was discussed in the Interactive Student Tutorial section of Chapter 7.13. Applied to metals, molecular orbital theory enhances our picture of metallic bonding. Let's consider a gaseous sodium molecule, Na2. According to MO theory, the 3s orbitals of the two Na atoms combine to give a bonding MO and a * antibonding MO. Each sodium atom has just one 3s valence electron, so the lower-energy bonding orbital is filled and the higher-energy antibonding orbital is empty. There are a large number (perhaps 1020) of sodium atoms in a sodium crystal, and the number of molecular orbitals formed is equal to the number of atomic orbitals combined.

As shown in the figure above, the difference in energy between successive MOs decreases and the energy levels merge into an almost continuous band (hence, the term band theory). Because each Na atom has one valence electron, the 3s band is half-filled.

Electrical potential can shift electrons from one set of energy levels to the other only if the band is partially filled. When a band is completely filled, there are no available vacant energy levels to which electrons can be excited. Materials that have only completely filled bands are electrical insulators. Metals can form conduction bands from s, p, or d orbitals.

Magnesium is a conductor because of the overlap of the 3s and 3p bands.

The maximum bonding for transition metals occurs around group 6B where a d band overlaps an s band to give a composite band consisting of 6 MOs per metal atom, half of which are bonding.

The metallic bonding activity allows you to compare metals and relate the number of valence electrons in each to the number of electrons in bonding and antibonding molecular orbitals. It also provides the melting points of the metals for the purpose of comparing the magnitude of interatomic attractive forces in each metal.

 Metallic Bonding

  1. Click on an element.
  2. Compare its melting point to the populations of bonding and antibonding orbitals.