Kossel-Lewis Approach to Chemical Bonding: As we know, the matter is made up of more than one type of element. Except for noble gases, no other elements, under normal conditions, can exist alone in nature. They occur as a group of atoms, in the form of one species, with characteristic properties. Since a group of atoms cannot hold themselves together, there is some force that is holding them together. This ‘attractive force’, which is holding the elements – atoms, ions and so on together to form different chemical species, is called a chemical bond.

Types and Requirements for a Chemical Bonding

However, a chemical bond is formed not only between similar atoms but even the dissimilar ones, and in different combinations, mostly unique in nature to the combining elements.  So, to answer questions like why atoms combine in unique combinations or why only some of them do, while the others do not, several theories and postulates were put forth by the researchers, such as:

1. Kossel-Lewis approach
2. VSEPR or Valence Shell Electron Pair Repulsion theory
3. Valence Bond theory
4. Molecular Orbital theory

Each of these theories brings more clarity to the chemical bonding and are related to a clearer understanding of the structure of the atom, periodic table, and the electronic configuration of the elements. Also, they all seem to agree on the fact that every system wants to attain stability, and they do that through bonding, which reduces the energy and brings in more stability to the system. The individual approaches or theories given to understand chemical bond formation slowly unravelled the mystery behind the combinations and how or why only some elements form bonds and that too, in unique ratios with other elements or atoms.

Kossel-Lewis Approach to Chemical Bond Formation

While several approaches were made before, in the year \(1916\), two scientists were successful in giving a satisfactory explanation to the formation of chemical bonds in terms of electrons. Kossel and Lewis made an attempt, separately and independently, to give a logical explanation behind the inertness of noble gases. The theory was based upon the electronic concept of atoms, and therefore, is referred to as the ‘Electronic theory of valency’. Kossel’s study was based largely upon inorganic compounds, while Lewis focused his studies primarily on organic compounds.

Electronic Theory of Valency – Kossel and Lewis Approach to Chemical Bonding

In his study, Lewis considered an atom as a positively charged ‘Kernel’, with a nucleus and the inner electrons. Also, the outer shell, according to him, could accommodate only a maximum of eight electrons. According to his assumptions, Lewis thought that the eight electrons occupy the corners of a cube, which then encircle the Kernel (the positively charged centre). Thus, according to his theory, while in Sodium or Potassium, only one corner of the cube is occupied by one electron, in a noble gas, all corners of the cube are occupied by 8 electrons, thereby leaving no space. The ‘OCTET’, therefore, ensures stability in the noble gases.  

So, Lewis postulates state that ‘all atoms achieve a stable octet, when they are linked by chemical bonds.’

At times, as in the case of Helium, the outer shell has only \(2\) electrons, and therefore, they are called the ‘duplet’. Hence, the concept of octet and duplet lead to the conclusion that when an atom or element has \(8\) electrons (or \(2\)), they attain a stable configuration and become chemically inert. On the other hand, elements like Sodium, chlorine, etc., can become stable only when, for example, sodium can transfer an electron to chlorine, thereby forming \({\rm{N}}{{\rm{a}}^ + }\) and chlorine can gain one to form \({\rm{C}}{{\rm{l}}^ – }\) and therefore, becoming stable, by attaining octet.

This concept of Lewis gave rise to the octet rule, which states that:

‘The atoms of different elements combine with each other to complete their octets or \(8\) electrons (or duplet or \(2\) electrons) in their outer shell.’

So, below-mentioned are a list of chemical combinations in which an atom can combine with one another.

a. Atoms can combine with each other by the complete transfer of one or more electrons from one to another. This process is called electrovalence, and the chemical bond formed is an ionic bond.
b. Atoms can combine by sharing electrons with each other, and this can occur either by equal sharing of electrons called a covalent bond, where both atoms contribute equally to form a bond.
c. A sharing of electrons where one atom contributes the electron and the other does not, but still, the atoms are shared between the two. Such a bond is known as a coordinate bond or a dative bond.

Lewis Symbols to Represent Valence Electrons

Lewis, in his observations, noted that in the formation of a molecule, only the outer electrons are involved, and therefore, they are called the Valence Electrons. The electrons inside the shell, or the inner-shell electrons, are protected and are not generally involved in the bonding or combination process. Hence, when discussing bonding, one needs to consider the outer shell electrons as active participants.

Lewis devised simple symbols through which it is easier to represent the valence electrons in an atom. Accordingly, the outer shell electrons are represented as dots surrounding the symbol of an atom. These symbols are called Lewis symbols or electron dot symbols. The symbols do not consider the inner shell electrons since they are not participating in the bond formation. Examples are as follows:

The noble gas, such as Neon, can be represented as:

Significance of Lewis Symbols

The representation of elements and the valence electrons around the symbol gives the exact number of electrons present in the outermost shell of an element. This number helps one calculate the valency of an element since the outer shell electrons are called valence electrons. Hence, these symbols play a vital role in determining the valency of an element.

The common valency of an element can be:

1. Equal to the number of dots on the Lewis symbols, for example, as in elements such as \({\rm{Li}},\,{\rm{Be}}\) and \({\rm{B}}\) (elements having valence electrons up to four).
2. Equal to \(‘8\) minus the number of dots in the Lewis symbols’ as seen in the case of \({\rm{N}},\,{\rm{O}}\) and \({\rm{F}}\) (elements having valence electrons more than four).

So, while the valency of \({\rm{Li}},\,{\rm{Be}}\) and \({\rm{B}}\), are \(1,\,2\) and \(3\), the valency of \({\rm{N}},\,{\rm{O}}\) and \({\rm{F}}\) would be respectively given as:

1. \(8 – 5({\rm{dots}}) = 3\) (Nitrogen)
2. \({\rm{8 – 6 = 2}}\) (Oxygen)
3. \({\rm{8 – 7 = 1}}\) (Fluorine)

Postulates of Kossel-Lewis Theory

Kossel put forth several facts regarding chemical bonding, and the significant postulates/facts include:

1. The highly electronegative halogens and the highly electropositive alkali metals are segregated by noble gases in the periodic table.
2. The formation of negative ion from halogens and positive ions from alkali metals are associated with gain and loss of electrons, respectively.
3. The negative and the positive ions formed attain stable noble gas configurations or electronic configurations.
4. All noble gases have a stable electronic configuration of an octet \(\left( {{\rm{n}}{{\rm{s}}^2},\,{\rm{n}}{{\rm{p}}^6}} \right)\), except helium which has a duplet configuration \(\left( {{\rm{n}}{{\rm{s}}^2}} \right)\).
5. Electrostatic attraction holds the positive and negative ions together and stabilizes them.

Example: In the formation of \({\rm{NaCl, Na}}\) loses an electron, which is gained by chlorine. In losing an electron, \({\rm{Na }}\) attains the stable configuration of Neon, while \({\rm{Cl }}\), on gaining an electron, gets the stable configuration of Argon.

\({\rm{Na}} \to {\rm{N}}{{\rm{a}}^ + } + 1{{\rm{e}}^ – }\)

\(\left[ {{\rm{Ne}}} \right] + 3\;{{\rm{s}}^1}{\mkern 1mu} \left[ {{\rm{Ne}}} \right]\)

\({\rm{Cl}} + 1{{\rm{e}}^ – } \to {\rm{C}}{{\rm{l}}^ – }\)

\(\left[ {{\rm{Ne}}} \right] + 3{{\rm{s}}^2}3{{\rm{p}}^5}\,\,\,\,\left[ {{\rm{Ar}}} \right]\)

The bond thus formed, by electrostatic attraction between negative and positive ions, was named as electrovalent bond. The electrovalence is equal to the number of ‘unit charge or charges on the ion’. 

Example: Sodium has an electrovalence of \(+1\), while chlorine has an electrovalence of \(-1\).

Significance of Kossel Postulations

Kossel’s postulates helped in:

1. Laying the foundation stone for present-day ion-formation concepts through electron transfer and also the formation of ionic crystalline compounds.
2. In understanding and systemizing ionic compounds.

Kossel also recognized the fact that several compounds do not fit into the category or concepts he underlined.

Summary

While some atoms or elements exist in nature in a combined form, in the form of compounds or molecules, some exist alone, like noble gases. The reason behind why elements combine in unique ways with others to form compounds has been researched widely to arrive at the present day explanation of a chemical bond. Kossel and Lewis were the first to come up with some form of explanation for the formation of chemical bonds, valence electrons and what they represent. While they researched and arrived at similar conclusions independently, each contributed to the basis of the theory of modern chemical bonding. Lewis gave symbols to help represent valence electrons around an atom, while Kossel put forth a postulate which explained why elements combine to attain a stable electronic configuration.

FAQs on Kossel-Lewis Approach to Chemical Bonding

Q.1. What is the basis of Lewis approach towards the formation of the chemical bond?
Ans: According to Lewis, elements combine with each other to attain octet or duplet, a stable electronic configuration as to that of noble gases. And, when they lose or gain electrons, a bond is formed.

Q.2. What is a chemical bond explain with reference to Kossel Lewis approach?
Ans:  Kossel and Lewis researched independently to arrive at a conclusion that the atoms or elements form an electrovalent bond when they gain or lose electrons to attain an octet (or duplet) in their outer valence shell.

Q.3. What type of chemical bond did Kossel and Lewis predict?
Ans: Kossel and Lewis both predicted the formation of an electrovalent bond or the ionic bond.

Q.4. What are the 3 types of chemical bonds?
Ans: The three types of chemical bonds are ionic bond, covalent bond and coordinate bond or dative bond.

Q.5. What is a chemical bond? Explain with examples.
Ans: A chemical bond is a force that holds the atoms together in a molecule or a compound. For example, sodium and chlorine combine to form \({\rm{NaCl}}\), and the bond between them is called an ionic or electrovalent bond because the sodium atom loses an electron while the chlorine atom accepts an electron to attain a stable electronic configuration and thereby forming the bond.

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