Metallic Bonding

More than 80 elements in the periodic table are metals. Metals are solids at ordinary temperature and pressure, with the exception (of mercury and gallium). Metals have characteristic properties such as:

• High thermal and electrical conductivity.

• Luster and high reflectivity.

• Malleability and ductility. They can be beaten or shaped without fracture.

• Variability of mechanical strengths (ranging from soft alkali metals to Tungsten, which is hard).

The force that binds together the atoms of metals is called metallic bond. The properties of metals cannot be explained in terms of common types of bonds such as ionic and covalent bonds. The inadequacy of these two type of bonds for metal formation can be explained as under.
The atoms of metals are all alike therefore they cannot form ionic bonds. Moreover, ionic compounds do not conduct electricity in the solid state and ionic compounds are brittle as opposed to properties of metals. The atom of metallic elements contain only 1 to 3 valence electrons, therefore these atoms cannot form covalent bonds, with noble gas configurations as they will remain incomplete. Covalent compounds are bad conductors of electricity and are generally liquids; properties opposed to metal formations. Thus, metals have a different model of bonding.

Electron sea model for metallic bonding

To account for the bonding in metals, Lorentz proposed a model known as electron gas model or electron sea model. This model is based on the following characteristic properties of metals:

Low ionization energies

Metals generally have low ionization energies. This implies that the valence electrons of metal atoms are not strongly held by the nucleus. Valence electrons can move freely out of the influence of their kernels (atomic orbit/structure minus valence electrons). Thus, metals have free mobile electrons.

Large number of empty orbitals

It has been observed that in metals a number of valence orbitals remain empty as the number of valence electrons in metals is generally less than the number of valence orbitals.
For example, lithium {(Li, Z = 3) 1s²2s¹} has 2p-orbitals vacant; Sodium {(Na, Z = 11) 1s²2s²2p⁶ 3s¹} has 3p-and 5d-orbitals vacant;
Magnesium {(Mg, Z = 12) 1s²2s²2p⁶ 3s²} has 3p-and 3d-orbitals vacant The important features of electron sea model are:

• The positively charged kernels of metal atoms are arranged in a regular fashion in a metallic lattice.

• Loosely held valence electrons, surround each kernel in metallic lattice. Being loosely held to its kernel, the valence electrons enjoy complete freedom in the metallic lattice and are regarded as mobile electrons.

In short, the metal may be regarded as 'a sea of electrons (common pool of electrons) in which there is a three dimensional ordered arrangement of positively charged kernels, surrounded throughout by mobile valence electrons'. This explanation is also responsible for its name electron sea model. Thus, the simultaneous force of attraction between the mobile electrons and the positive kernels that binds the metal atoms together, is known as metallic bond.

Fig: 6.19 - Electron sea model

Comparison of Ionic bond Covalent bond and Metallic bond

Ionic Bond	Covalent Bond	Metallic Bond
The transfer of electrons between two atoms having different electro negativities forms this bond.	This bond is formed by the mutual sharing of electrons between same or different elements .	This bond is formed due to the attraction between kernels and the mobile electrons in a metal lattice.
This is a strong bond due to electrostatic force of attraction.	This is also a fairly strong bond because the electron pair is strongly attracted by two nuclei.	This is a weak bond due to the simultaneous attraction of the electrons by a large number of kernels
This is a non-directional bond.	This is a directional bond.	This is a non-directional bond.
This bond makes substances hard and brittle.	This bond makes substances hard and incompressible.	This bond make substances malleable and ductile.

Explanation of physical properties of metals

All the characteristic metallic properties can be explained on the basis of the electron.

Metallic lustre

The bright lustre of metals is due to presence of delocalised mobile electrons.
When light falls on the surface of the metal, the loosely held electrons absorb photons of lights. They get promoted to higher energy levels (excited state), oscillating at a frequency equal to that of the incident light. These oscillating electrons readily return from the higher to the lower levels of energy by releasing energy, thus becoming a source of light radiations. Light appears to be reflected from metal surface and the surface acquires a shining appearance, which is known as metallic lustre.

Electrical conductivity

The presence of mobile electrons causes electrical conductivity of a metal. When a potential difference is applied across the metal sheet, the free mobile electrons in the metallic crystal start moving towards the positive electrode. The electrons coming from the negative electrode simultaneously replace these electrons. Thus, the metallic sheet maintains the flow of electrons from negative electrode to positive electrode. This constitutes electrical conductivity.

Thermal conductivity

When a part of the metal is heated, the kinetic energy of the electrons in that region increases. Since the electrons are free and mobile, these energetic electrons move rapidly to the cooler parts and transfer their kinetic energy by means of collisions with other electrons. Therefore, the heat travels from hotter to cooler parts of the metals.

Malleability and ductility

Metals can be beaten into sheets (malleability) and drawn into wires (ductility). Metallic bonds are non-directional in nature. Whenever any stress is applied on metals, the position of adjacent layers of metallic kernels is altered without destroying the crystal. The metallic lattice gets deformed but the environment of kernels does not change and remains the same as before. The deforming forces simply move the kernels from one lattice site to another.

Fig: 6.20 - Displacement of metal kernels in a metallic lattice

High tensile strength

Metals have high tensile strength. Metals can resist stretching without breaking. A strong electrostatic attraction between the positively charged kernels and the mobile electrons surrounding them is the reason for tensile strength.

Hardness of metals

The hardness of metals is due to the strength of the metallic bond. In general, the strength of a metallic bond depends upon:

• The greater the number of valence electrons for delocalisation the stronger is the metallic bond.

• Smaller the size of the kernel of metal atom, greater is the attraction for the delocalised electrons. Consequently, stronger is the metallic bond.

For example, alkali metals have only one valence electron and larger atomic kernels, which makes the metallic bonds weak. Consequently these metals are soft metals.

Opaqueness

The light that falls on metals is either reflected or completely absorbed by the delocalised electrons. Because of this, no light is able to pass through metals and they are termed as opaque.

Melting and boiling points

Metals have metallic bond strengths, which is intermediate to that of covalent and ionic bonds. Therefore in general, metals have boiling and melting points in between to that of covalent and ionic compounds.