Metallic Bond: Definition,Properties,Examples,Electron Sea,Band Theory,Diagram

Metals are believed to possess a special type of bonding known as Metallic Bond. There are only few valence electrons or outermost electrons are very loosely bound to the nucleus.This develops the ability in metals to conduct electricity and heat. In the metallic bond, most metals crystallize in close- packet structures. There is a strong electron interaction among 8 to 12 nearest neighbour atoms also called as coordination number. Metallic crystals have considerably weak strength. So, the forces like van der Waal forces cannot seem to exist between the metals.

What is Metallic Bond- Definition

The simultaneous attractive interaction between the mobile electrons and cores constitute a weak bond which is known as Metallic Bond.

This theory was proposed by Lorentz in 1923 and it is based on the following properties. This concept is based on the idea that high electrical and thermal conductivities of metals are due to the presence of free electrons (or mobile electrons). To understand this model of the metallic bond, the following characteristic properties of individual atoms of the metals must be remembered. 

  • The ionization energies of the atoms of the metallic elements are quite low. This means that electrons present in the valence shell of metals atoms are loosely held by the nucleus.
  • The number of valence electrons in metallic atoms is less than the number of vacant orbitals (empty valence orbitals).
Metals Empty Valence orbitals.
Li (lithium: 1s2, 2s1, 2p0) Three 2p orbitals are vacant.
Na (Sodium: 1s2,2s2,2p6,3s1,3p0,3d0) Three 3p and Five 3d Orbitals empty.

Thus, the metals atom has only a few electrons in their valence orbitals (<1 or 2 in most of the cases). On account of the low ionization energies of metals have their valence electrons are loosely bound to the Kernels.

Kernels: Nucleus and all other bounded electrons except those in the valence shell.Metallic Bond

Consequently, some of the atoms lose one or more of the valence electrons and change into positive ions (cations). When these electrons move. This movement of mobile electrons within the metal is similar to the movement of molecules in gases. Due to this reason, this concept is known as electron gas model.

Electron Sea model or Electron Gas Theory

According to the electrons sea model, the metals are regarded as a sea of negatively charged valance electrons in which positively charged kernels are immersed or dipped. In the three-dimensional space model for the metal, each kernel is surrounded by a number of valence electrons and vice versa.(see above diagram)

The Valence electrons are mobile and can move in the electron sea from one place to the other. On the other hand, the kernels are heavy and have comparatively very little mobility. Thus, the Mobile electron valence electrons are mobile or delocalised. Their movement in the electron sea is just like that of the gas molecules. Therefore, the theory is called electron gas theory.

Thus, Metals have the ability to conduct electricity and heat. If metals ends are connected to the external source of electric current. Free electrons start moving from one end and cross through metal.  The rate of conductivity of electrons will be the same. These electrons move freely and randomly throughout the metal which may be regarded as a collection of positive ions (cores or kernels) immersed in a sea of mobile electrons. In Lithium,  the ions would be Li+ and one electron per atom would contribute to sea. These free electrons account for the characteristic metal properties.

Metallic Bond

Limitation of Electron Gas Theory

Although the electron gas model has successfully explained most of the physical properties of the metals but it fails to account for the large variations in them. For example, the melting point of mercury (134 K) is very small as compared to that of tungsten (3575 K). Similarly, copper has been found to be at least 50 times more conducting than bismuth.

 Band Theory of Metals Or Molecular Orbital Theory

The bonding in metals can also be explained on the basis of molecular orbital theory. This theory also called as band theory. This was developed by Helix Block in 1928.

Remember that

  • In the combination of one atomic orbital belonging to two atoms, two molecular orbitals are formed. Out of these, one is bonding molecular orbital while the other is antibonding molecular orbital.
  • In case, atomic orbitals belonging to three atoms are involved, then three molecular orbitals result.
  • In addition to bonding and antibonding molecular orbitals, there is another molecular orbital called non-bonding molecular orbital. The non-bonding molecular orbital lies in between bonding and antibonding molecular orbitals and has exactly the same energy as the participating atomic orbitals.

Letus illustrate by lithium (1s2s1) as follows:

Note that the non-bonding molecular orbital has no contribution towards the energy state of the molecule.

Now let us extend this theory and consider the combination of four atomic orbitals belonging to four atoms of lithium. As expected, four molecular orbitals will emerge out of which two will be bonding and the other two will be antibonding.

From the above picture, it is quite evident that the lower bonding molecular orbital is slightly more bonding than the other. The same is true for the antibonding molecular orbitals also. In other words, the different bonding or antibonding molecular orbitals are not at the same energy state. We are aware of the fact that a large number of metal atoms are closely packed in space. Thus, in case ‘n’ atomic orbitals of the atoms take part in the bond formation then ‘n’ molecular orbitals will result. These will be so close to energy that they begin to blur and will not present a distinct view. A continuous energy band involving a number of such molecular orbitals will be the outcome.

Explanation of Conductor, Insulator and Semi-Conductors

According to the band theory, When energy is supplied to electrons. They jump from the valence band to conduction band. The conduction band allows electrons to move freely.This cause the flow of current. The electrical conductivity of metal decreases as temperature increases. The increase in temperature causes thermal agitation of metal ions.This impends the flow of electrons when an electric field is applied.

  • Crystalline nonmetals, such as diamond and phosphorus are insulators. They do not conduct electricity. It is due to the fact that there is highest energy electrons occupy filled bands of molecular orbitals that are separated from the lowest empty band (conduction band) by the energy difference called the band gap.In insulators, this band is in energy different that is too large for him to jump to get the conduction band.
  • Elements that are semiconductors have filled bands that are only slightly below.They do not overlap with the empty band. They do not allow electricity to pass at low temperature. A small increase in temperature sufficient to excite some of the electrons. The electrons jump into the highest energy band conduction band.

Metallic Bonding Diagram

The diagram shows the movement of electrons in a metal. The movement is non-directional. The strength of metallic bond decreases with:

  • The Increase of valence electrons.
  • The decrease in the size of the atom.

Properties of Metallic Bonds

The different Properties of Metals can be explained with the help of electron sea model.These are discussed as follow:

Electrical conductivity

The high electrical conductivity of metals is because of the presence of mobile valence electrons which are ordinarily flowing probably equally in all directions. When a potential difference is applied across a metal, the electrons start moving towards the positive end. Consequently, the electric current flows throughout the metal. The electrical conductivity decreases with the rise in temperature.

Thermal conductivity

The high thermal conductivity of metals is also due to the presence of mobile electrons. On heating one end of a metal, the kinetic energy of the electrons of that end increases. Consequently, the electrons start moving rapidly to another end i.e. cooler end of the metal and transfer some of their energy to this end. As a result, this end also acquires higher temperature so that the heat gets conducted throughout the metal.

Metallic lustre

When a beam of light falls on the surface of a metal, the surface electrons absorb photons of light and are set into to and fro oscillations. These oscillating electrons, being charged, immediately emit the absorbed energy in the form of visible light.  As such, the light falling on the surface of metal appears to be reflected giving rise to shining appearance known as metallic lustre. In all metals, except copper and gold, the electrons absorb light of all wavelengths in the spectrum.

Malleability and Ductility

Metals can be hammered into thin sheets (malleability) and drawn into wires (ductility) because of the non-directional nature of the metallic bond. When an external stress is applied, the layers of metal ions slide over one another but the basic structure remains the same because mobile electrons adjust rapidly to the new situation. Thus the shape of the metallic crystal can be changed or deformed without breaking.Metallic Bond

Tensile strength

Metals have a high tensile strength, i.e., they can resist stretching without breaking. Thus a large weight can be supported even by a wire of small cross section. The high tensile strength is due to the strong electrostatic attraction between the positively charged metal ions and the sea of mobile electrons.

Example of Metallic Bond

This picture of positive ions immersed in a sea of electrons is independent of directional limitations i.e., the metallic bond is not directional. By hammering the internal structure remain unchanged as a sea of electrons rapidly adjust to the new situation. Metallic Bond does not include the bond formation of elements which participate in covalent bonding. 

Examples of metallic bond include all metals such as Copper(Cu), aluminium (Al), silver(Ag), gold(Au). Whereas the d block elements comprised the metallic bonding as well as Covalent Bonding. The reason behind of two types of bonding is involvement of 3d orbitals electrons.

Ionic Covalent and Metallic bonds

Mobile View: Full Table

 

 

 

Characteristics Ionic Metallic Covalent
Constituent Particles Positive and Negative Ions Positive ions in a sea of electrons Atoms
Binding Force Electrostatic Attraction Between Ions Electrostatic Attraction Between cations and sea of electrons Strong Covalent Bond
Hardness Very Hard Variable Hard Except for graphite
Brittleness Shiny Very Low Medium
Melting Point Very High Moderate High
Conductivity Conduct in  Molten State Good Conductors Bad Conductors Except for Graphite
Solubility Soluble in Polar and Non-Polar Insoluble in Polar and Non-Polar Insoluble in Polar and Soluble in Non-Polar Solvent
Examples NaCl, ZnS, CaO, KNO3 etc. Metals and Alloys Diamond Graphite, Sulphur, SiC etc.

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