Metallic bonding, structure of metal and theories of metallic bonding

 In this article, I will discuss metallic bonding.

Metallic bonding

       Metallic bonding is present in metals. Metal is basically electropositive elements present at the start of the periodic table and at the last positions of groups. Metals are those substances that have a bright appearance; this is known as metallic luster. Metals are good conductors of heat and electricity.

Structure of metals

       Metals have a very high coordination number, mostly twelve or eight. Metals have very electrical and thermal conductivities. Metal atoms are packed in such a way that, they build a crystal.

       Metals are considered assemblages of positive ions in the form of sphere of identical radius, which is packed in such a way to fill the maximum possible spaces. Most of the metals have three types of crystal structures;

·      Body-centered cubic

·      Face centered cubic

·       Closely packed hexagonal structure

In a body-centered cubic structure, each atom of metal is surrounded by eight other atoms, this is known as 8-fold coordination. In a face-centered and closely packed hexagonal structure, each atom of metal is surrounded by 12 other atoms, this is known as 12-fold coordination.

metallic bonding

Theories about metallic bonding

       There are many theories about metallic bonding.

Electron gas theory

Pauling theory

Band theory

Bloch-sommerfeld theory

Electron gas theory

       Metals have the ability to lose electrons, so valence electrons of metals are free to move in space in metallic structure. Picture (2) shows the sea of negatively charged around the positive ion. Both positive and negative charges are held together tightly. There are three assumptions of this theory;

•   Metal atoms are arranged in such a way that to make a crystal lattice. Each metal atom in a crystal lattice loses one or more electrons, as a result, charged metal ions are produced that are known as atomic cores.
•  The entire electron moves freely within the boundaries of the metal structure.
• . There is a coulomb attractive forces exist between metallic ions and electrons. These forces are like electronic glue.

1. his theory shows that metals have high melting points and high electrical conductivities due to free electrons. The metals have shiny surfaces due to their high reflectance.

structure of metal

Pauling theory

       This theory is given by Pauling. According to this theory, the structure of the metal is considered in terms of covalent bonds which resonate among interatomic positions in metals. Due to resonance, a different covalent bond is formed than the expectation. Due to resonance different atomic orbitals combine with each other and form molecular orbitals. This theory is also known as the valence bond theory.

resonance structure

Band theory

       This theory is also known as molecular orbital theory. According to this theory, when two-atom come close to each other a mutual perturbation of orbital is generated as a result two new molecular orbitals are formed with different energies. One of the newly formed molecular orbitals is known as bonding molecular orbitals and another orbital is known as anti-bonding molecular orbital.

molecular orbitals

Example

       If four identical atoms like A, B, C, D combine with each other, then atomic orbitals give rise to one bonding and one anti-bonding molecular orbital.

Bloch-sommerfeld theory

       This theory is initiated by Sommerfeld in 1928 and later on, Bloch extend this theory and give the following postulates.

1.  The metal consist of atomic cores which are closely packed. A neutral atom loses those electrons from the outermost shell which are loosely bound and these electrons are called valence electrons.

2.  The positive cores have an area of negative potential within the volume occupied by the metal. This potential is higher outside the metal and zeroes within the metal.

3.  All the electrons of metal are confined only in the interior of metal because outside of the metal potential is very high. All the electrons obey the requirements of quantum mechanics.

According to this model, electrons are restricted to a certain permitted band called the Brillouin zone. These bands are also called energy bands.