Introduction

All the things we see and feel, even the air we breathe are altogether known as ‘material.’ Through centuries of scientific investigation we have come to know and understand that all material is composed of atoms and atoms are in turn made up of nucleus and electrons. The number of electrons surrounding the nucleus defined as atomic number dictates the characteristic of that atom. There are more than 100 different types of atoms on earth and they are systematically arranged in Mendeleeve’s chart, or known as the ‘PERIODIC CHART OF THE ATOMS.’

We also know that atoms are held together through electrons in forming material. Our experience convinces us that there are two types of electron bonding: a) strong bonding and b) weak bonding. The weak bonding does not involve direct electron-electron interaction but rather utilizes Van der Waals’ forces or dipole-dipole interaction in forming bonding. The strong bonding, on the other hand, involves direct electron-electron interaction and can be differentiated into three groups: Covalent bonding, Ionic bonding and Metallic bonding.

COVALENT BONDING; involves a pair of electrons with opposite electron spin. The bond (or electron charge distribution) is essentially localized between nearest neighbor atoms that contribute electrons for the bonding. Since these electron pairs follow ‘Bose-Einstein’ statistics, therefore they are known as ‘boson.’ In this case the paired particles do not obey the Pauli Exclusion Principle and many electron pairs in the system may occupy the same energy level.

IONIC BONDING; also involves a pair of electrons with opposite electron spin. However, the bond (or electron charge distribution) is lopsided on one atom. And results in negatively charged atom on one and leaving the other atom positively charged. This creates a negative-positive charge attractive force that binds atoms together. Statistically, these particles are also ‘Boson.’

METALLIC BONDING; in this bonding, electrons are not paired and are quasi-free to roam throughout the system. Because of unpaired electron spin, they follow the ‘Fermi-Dirac’ statistics and consequently obeying the Pauli Exclusion Principle. Therefore they are known as ‘fermions’ where no two electrons can occupy the same energy level and results in an energy band. The three distinct type of bonding as described above is too abstract and difficult to follow without a Quantum-Mechanical upbringing. The understanding of these bonding may be enhanced by an illustration through an imaginary ‘dog show’ as follows.

Let us imagine that there is a dog-show to be held and people coming to the show are all exactly identical to one another in all respect. These persons are to be known as nucleus (or nuclei as you prefer). Each person is allowed to bring three dogs, which are to be called ‘electron’. Naturally, the dogs (electrons) are all identical to one another and will be on a leash. The leash will be known as ‘atomic orbital’ and be held firmly by each nucleus. As long as the nucleus holds tightly on the leash the dogs (electrons) will not run away. This unit, namely the nucleus (person) and electrons (dogs) will be known as an atom. Now, as the atoms with their nuclei and their dogs (electrons) come close to one another heading to the dog-show, the dogs (electrons) from different nuclei will be barking (interact) at one another – the closer they are, the more vigorous will be the barking (interaction). Ultimately the atoms are hurdled into the space where the dog-show is to take place and the atoms come so close to one another that the dogs (electrons) not only would bark at one another but also would actually chase one another. With this picture in mind we can now illustrate the three distinct types of bonding in the following way:

Covalent Bond: a pair of dogs (electron), each comes from two adjacent nuclei (person), chase one another in a round-and-round way so that their leashes are twisted together (Fig. 1). As long as the person (nucleus) continues to hold onto their leash the two adjacent nuclei are tied (bonded) to one another through the twisted leash and the pair of dogs (electron). This is the picture of covalent bond and otherwise known as linear-combination-of-atomic-orbital (LCAO).

Ionic Bond: it involves also a pair of dogs (electron) from two adjacent nuclei (persons) and having their leashes twisted together in the same way. The one big difference is that there are two distinct nuclei (persons) instead of identical nuclei (people). For example, a man-nucleus and a woman-nucleus and assume the man-nucleus is stronger than the woman-nucleus. The formation of a twisted pair of electron (dog) between man-woman-pair of adjacent nuclei will take place much in the same manner as in all identical nuclei. However, in this case the weaker woman-nucleus may let go the leash and results in paired electron (dog) being pulled to the man-nucleus side (Fig. 2). This results in man-nucleus with an extra electron (dog) while woman-nucleus short of one electron (dog). Thus, there is no direct leash tie to hold man-woman-nuclei pair together. Flowever, the woman-nucleus would not leave because she sees her dog (electron) being held by the man-nucleus and cannot leave. Thus, the formation of positive-negative charged ionic bonded pair. It should be stressed therefore the ionic bond formation requires two distinct types of nuclei.

Metallic Bond: here we assume that each nucleus (person) is already leash-tied (covalent-bonded) to its two neighbors through leash-twisted pair of dogs (electrons) and therefore is hands-full. Under this circumstance the nucleus may not be able to hold onto the leash holding the remaining electron (dog) such that the electron (dog) may pull away from the nucleus (person) and become free to roam within the building confine. These dogs (electrons) running free are the so-called quasi-free electrons. The total number of dogs (electrons) vs. total number of people (nuclei) within the building remains the same with free-roaming dogs (electrons) providing cohesive attractive force to hold the dog-show attendants together.

Within these understanding, some specific feature of each bonding can be described. Covalent bond involves two and only two electrons. The nuclei that contribute electrons in forming the bond may or may not be the same type of atoms. Inasmuch as the bond is directly tied to the leash held by the nucleus the bond is necessarily directional and very strong and confined to the space between the two nuclei forming the bond. Since the bond requires a tight grip of the leash by the nucleus, the number of bonds can be formed by a given nucleus is therefore limited according to the strength or available atomic orbital of the nucleus.

Similarly, Ionic bond also involves electron pair but because the weaker nucleus gives up the leash entirely the bonding is not as strongly directional as well as in strength as in Covalent bond. Because the bonding is electrostatic in nature the bonding forces may involve not only the nearest neighbors but also the second, third, fourth and even higher order of neighboring attractive and repulsive forces and known as Madelung energy. Metallic bond involves all quasi-free electrons that are running free among nuclei and within the confine of the space. Therefore the bonding is totally non-directional and delocalized. They are considered as weak bonds. Due to the dynamic nature of these electrons they are treated as waves traveling in a space of periodically placed positively charged nuclei. These waves are known as Bloch-wave function. The periodic nature of nuclei distribution creates Brillouin zone boundary, which interact with the traveling Bloch waves. The bond created in this manner is found mostly in metallic elements and contributes to the conduction of electricity.

With the characteristic of each bonding type identified, we shall now show how all the material in this universe can be assigned to a particular type of bonding or a combination of bonding type as summarized in Fig. 3.

In Fig. 3, a solid line ties covalent bond and ionic bond to another. This is to show that both the covalently bonded and the ionic bonded electrons are both ‘bosons.’ And quantum-mechanically they are equivalent to one another. At two extremity, i.e., 100% of covalent bond can be represented by a material known as diamond, whereas 100% of ionic bond can be represented by a material known as table-salt, Na(+)C1(-). All the organic and inorganic material in this universe can be assigned between these two extremities. The difference, from one material to another, lies only in terms of the percentage of covalency or ionicity.

It is clear that ionic bonded material is an insulator and metallic bonded...