Inorganic Chemistry/Chemical Bonding/Introduction
There are more than 110 elements, known so far, in the world of Chemistry. All these elements combine with each other, under certain conditions, to give rise to compound particles called molecules or simply compounds. The process of formation of these compounds, by combination of two or more atoms of the same or different elements to form molecules, is known as Chemical Bonding. The result of this phenomenon is called a Chemical Bond. Throughout Inorganic Chemistry, the term bond may be presumed to be synonymous with force. Hence, a Chemical Bond is defined as the strong force of attraction which can hold a group of two or more atomic species together. The atomic species, here, can be referred to, more or less, the atom itself. A single bond binds two atoms, though multiple bonds may exist between two atoms. A Chemical Bond is defined as the force of attraction which can hold a group of two or more atoms together. A single bond binds two atoms. More comprehensively, a Chemical Bond can also be defined as a force of attraction which holds together a group of two or more different types of atom in such a way that one chemical bond gives rise to a diatomic unit which is itself the new molecule or is a part of the newly formed species called a molecule or a compound. For example, in a water molecule, where the oxygen atom is bonded with two hydrogen atoms, the O-H bond acts as a unit(water molecule has two such units). Hence, a chemical bond is the force which results in such a unit. Units like these, similar or dissimilar, will combine with each other, accordingly, again with the help of the chemical bonds, to give rise to a compound. The properties of this compound generally differ from those of the individual elements involved in bonding.
All elements combine with each other, according to the way their properties permit them to bond, by means of these chemical bonds giving rise to more than 7 million known compounds. This is the same as the way in which a few letters of the alphabet of a language are arranged in different patterns to give rise to innumerable words and in turn form many sentences. Just as grammar gives the rules which govern the formation of the words and sentences in the language, there are also conditions which govern the formation of chemical bonds between different elements. These conditions, for each type of bond, will be discussed in detail in further topics. But after knowing what a chemical bond means, we have to explain the basic factor for chemical bonding: Why does Chemical Bonding occur? What might be the reason that the atoms combine with each other by means of chemical bonds? Another question which arises is that when the atoms are neutral in their ground state, what is the agent which is responsible for the force, called the bond, between them? In other words: How does the chemical bond occur? The last fundamental question is: How can the changed properties of the compound be determined by observing the bond formed? These three are the fundamental questions of chemical bonding. Listing them again:
- Why does Chemical Bonding occur?
- How does the chemical bond occur?
- How can the changed properties of the compound be determined by observing the bond formed?
Before attempting to answer each of the above questions, we have to get to know an important characteristic feature of atoms. And this feature is called Valency or Valence. Though this concept evolved in the 18th Centaury, the first theory of valency was put forward by Edward Frankland in 1852. The word Valency is derived from a Latin word Valencia which means capacity. This implies that it is the combining capacity of the atoms. Therefore, Valency of an element was defined as the number of Hydrogen or Chlorine atoms with which an atom of that element combines, to form a compound. The valencies of H and Cl were concluded as one. For example, since Oxygen combines with two Hydrogen atoms to form a water molecule, the valency of Oxygen was stated two. But this way of evaluating valency gave variable and fractional valencies. The valencies of Nitrogen in the compounds NH3 , N2H4 and N3H were 3, 2 and 1/3 respectively. Hence this would add more confusion and therefore, the definition of valency was concluded as the number of chemical bonds the atom of an element can form in a molecule or compound. But even this gave variable valencies, not addressing the confusion. But after the discovery of a negetively charged elementary particle called electron, the theory of valency was finely innovated. This theory is called Electronic Theory Of Valency. It states the fact, which we now know, that electrons are responsible for the chemical bonding. According to this theory, valency is the number of electrons present in the outermost energy shell of the atom. This energy shell is called valency shell. The theory, in detail, shall be discussed after addressing the first fundamental question of chemical bonding.
Causes of Bonding
Chemical Bonding, as discussed earlier, is some interactions between a group of two or more atoms of similar or dissimilar elements. But we have to amicably agree that a this doesn’t happen just for the sake of formation of compounds. The formation of these Compounds is purely unintentional and their use by man is, to the same rate, incidental. The reason for these interactions is quiet natural.
Overall decrease in energy
Every system in the universe has a tendency to stay in the lowest energy form. This is unanimously an axiom. Since an atom is also a natural system, even it has a tendency of acquiring the sate of possessing the lowest potential energy. It is only under this condition that the atoms combine together. This means that the fundamental reason for the bonding of atoms is that they loose their stored potential energy by forming bonds. This is the only reason for chemical combination. Now we have to understand how does the energy, stored in the atoms, decrease due to formation of chemical bond? This needs some knowledge in the Work-Energy Theorem. According to this theorem, the work done on a system is equal to the change in the Kinetic Energy of the same system.
We also know that Work is equal to product of Force and displacement, provided that both the these vectors are in same direction. Or we can also say that Work is the dot product of Force and Displacement vectors.
Equations (1) and (2) imply that the product of force and displacement is equal to change in kinetic energy of the system.
But according to the Law of Conservation of Energy, positive change in kinetic energy is equal to negative change in potential energy.
Hence, as the dot product of force an displacement increases, the potential energy decreases. And this rule explains the interactions between the atoms. For example, If two atoms come close together, three types of attractive and repulsive forces come into play. They are:
- The force of attraction between the electrons(of both the atoms) and the nuclei of both the atoms.
- The force of repulsion between the nuclei of both the atoms.
- The force of repulsion between the electrons.
If the resultant of the work done in these three forces is positive, then there is a decrease in the amount of stored potential energy of the atoms. As a result, both the atoms prefer to go on interacting with each other as long as no external agent interferes this interaction. If the work done in the above three forces is negative, then there is less possibility of bond formation. Remember that the decrease in energy results in just interaction between the atoms and this interaction cannot be justified as a bond because these interaction easily fall prey to external disturbances. Hence they are not much stable. A bond is something which holds the atoms more strongly, to give rise to a compound which is more stable. It should, as well, be noted that the ultimate reason for bonding of atoms is always decrease in energy.
Attainment of stable configuration
As said above, the atoms combine with each other only when there is a decrease in stored potential energy. And a factor which contributes to the amount of this stored energy, in an atom, is its electronic configuration. In 1916, world famous chemists Gilbert N. Lewis and Walther Kossel, independently, found that the atoms of elements belonging to the last group of the periodic table, the Noble gases, have minimum amount of potential energy, under ordinary conditions. This was found to be due to the electronic configuration of their outermost energy shell or valency shell. In that case, they need not bond with any other atoms, implying that they are chemically inert. So any other atom, irrespective of its atomic number, whose valence shell has a configuration of the outermost shells of these Inert Gases, is said to have minimum energy and hence more stable. This is called Octet Rule. Except Helium, all the other Inert Gases’ outermost shells have a configuration of ns2, np6. Helium’s outermost shell’s configuration is just ns2. However, since the valency shell of Helium has no p-orbital, 2 electrons in outermost shell is acceptable to stability. It is only in case of absence of p-orbital that this applies. Therefore, the atoms whose outermost shells have a configuration of ns2, np6 or ns2(in special cases) are stable. The reasons for their stability in this state is too early to discuss. So, in this level, we have to assume it as an axiom.
Hence, the atoms always tend to attain this configuration of Noble Gases. They do this by interacting with other atom(s) and by forming bond with them. The mechanism of this bond forming answers the second fundamental question of chemical bonding. It shall be well explained after we learn about theElectronic Theory of Valency. By this, we understand that the arrangement of electrons in the atoms is responsible for chemical bonding. Hence, electrons are indirectly responsible for chemical bonding. It should be noted that octet rule is not always followed. There are some exceptions which are explained in Faliure of Octet Rule.
Electronic Theory of Valency
The Electronic Theory of Valency was the theory which gave valency, a uniqueness. It actually evolved due to the discovery of electron. Since it is a theory which relates valency with electrons, it is called Electronic Theory of Valency. This theory was developed by Gilbert N. Lewis and Walther Kossel. As we know, they found that Noble Gases are inert due to the configuration of their outermost energy shell. They have 8 electrons in their outermost shell, with exception to Helium. This implies that the number of electrons in the outermost shell is related to the stability of the atom. In other words, the electrons are themselves responsible for chemical bonding. Hence this theory was developed. This theory also explains how, actually, are the bonds formed. The main points of this theory are:
- Valency means the number of electrons in the outermost energy shell of the given atom. This outermost shell is called Valency Shell.
- The elements which have ns2, np6 or just ns2(in special cases) as the configuration of their valency shell are chemically inert, under ordinary conditions. They have minimum stored energy and need not form any bond. These are called Noble Gases.
- The atoms which does not have the said valency configuration are not much stable. They always tend to attain the configuration of the Noble Gases, i.e. have 8 electrons in the valency shell, and attain stability.
- The above said valency configuration can be attained by interacting with other atom(s). These interactions are of two types:
- Gaining or loosing electrons in such a way that at the end, both the atoms have 8 valence electrons.
- Sharing of the electrons between the atoms. (Note: Electrons are shared in pairs only)
This gaining or loosing or sharing of electrons, by an atom, are interactions which result in attractions between the atoms. Therefore, these interactions can be stated as bonds. The interaction which involves gaining or loosing of electrons is called Electrovalent Bond and the one in which sharing is done is called Covalent Bond. A comprehensive catagorization of these types of bonds shall be discussed in the next section. Now, the Electronic Theory of Valency can be concluded as: The union of two or more atoms occurs in such a way that the electrons are rearranged(either by transfer or sharing of electrons), so as, for both of them, to attain Noble Gas configuration of minimum stored energy.
Formation of a Chemical Bond
Now that we know that electrons are responsible for chemical bonding, we will be able to answer the second question. A chemical bond is formed by the interactions, between two atoms, which involve rearrangement of electrons in the interacting atoms. This rearrangement may be due to either transfer of electrons or sharing of electrons. These rearrangements result in forces which hold the interacting atoms, together. These forces are called bonds. There are many types of bonds. First, we will learn about the basic three types of bonds. Of these, two are nearly alike. They are:
- Electrovalent Bond
This is the bond formed due to transfer of electron(s) form one atom to another. This transfer results in formation of opposite charges on the atoms. These charged atoms(called ions) attract each other, forming the bond.
- Covalent Bond
In this bond the interacting atoms share electron pair(s). But both the atoms should contribute equal number of electrons in such a way that both of them attain octet configuration.
- Co-ordinate Bond
This is like a covalent bond. However, here the shared electron pair is contributed by only one atom. In other words, one atom contributes two electrons while the other contributes none, for sharing.
Apart from these, another type of bond is the Metallic bond. In this a metal shares its electrons among all the atoms. We will learn about all these types of bonding, in detail, in the following chapters. We will introduce more theories of chemical bonding in due course.
The Lewis Symbols, olso known as Lewis Dot Structures, are the structures which represent an atom of any given element. They are named after Gilbert N. Lewis, who introduced them in his 1916 article The Atom and the Molecule. These structures are extensively used to represent symbolically the valency (number of electrons in the outermost shell) of a given atom. These symbols basically give us an idea of the number of unpaired electons in the valency shell of the atom. They are represented by the symbol of the element, surrounded by either dots or crosses. The maximum number of dots for any element is taken as 8, because the valency of an atom cannot exceed 8. For writing these structures for an atom, we primarily need to know the exact electronic configuration of the given atom. Then we have to count the number of electrons in the outermost shell/valency shell of the atom. This number of electrons in the outermost orbit gives the valency of the atom. This many dots or crosses are placed around the symbol of the elemente.
For example, Chlorine has 7 electrons in its outermost shell. That means its valency is 7. Then the Lewis symbol of Chlorine is represented by putting 7 dots or crosses around its symbol(Cl), as shown in the adjecent figure. In this way we find that Cl has an unpaired electron. We can similarly draw Lewis structures for atoms of any element. Lewis Structures can also be drawn for the atoms which are bonded. These aspects will be discussed in a separate chapter.
The third question of chemical bonding is:How does the nature of bond determine the properties of the compound formed? The answer to this question mainly depends on the type of bond between atoms. Hence, we can in detail understand the characteristics of compounds, depending upon the type of bond involved in its formation. However, there are properties of compounds which are not explained by the bond type. In order to explain them, many more theories of chemical bonding were proposed. We would learn them in due course.