`=>` Matter is made up of one or different type of elements.

`=>` Under normal conditions no other element exists as an independent atom in nature, except noble gases.

`=>` However, a group of atoms is found to exist together as one species having characteristic properties. Such a group of atoms is called a `color{red}("molecule")`.

● Obviously there must be some force which holds these constituent atoms together in the molecules.
`color{purple}(✓✓)color{purple} " DEFINITION ALERT"`

`color{green}["The attractive force which holds various constituents (atoms, ions, etc.)"]`
`color{green}["together in different chemical species is called a
chemical bond"]`.

`=>` Since the formation of chemical compounds takes place as a result of combination of atoms of various elements in different ways, it raises many questions like :

● Why do atoms combine?

● Why are only certain combinations possible?

● Why do some atoms combine while certain others do not?

● Why do molecules possess definite shapes?

The answers to these questions are given by Kössel-Lewis approach, Valence Shell Electron Pair Repulsion (VSEPR) Theory, Valence Bond (VB) Theory and Molecular Orbital (MO) Theory.

`=>` In order to explain the formation of chemical bond in terms of electrons, in 1916, Kössel and Lewis succeeded independently in giving a satisfactory explanation.

● They were the first to provide some logical explanation of valence which was based on the inertness of noble gases.

`color{green}("Postulates :")`

(i) Lewis pictured the atom in terms of a positively charged ‘Kernel’ (the nucleus plus the inner electrons) and the outer shell that could accommodate a maximum of eight electrons.

(ii) These eight electrons occupy the corners of a cube which surround the ‘Kernel’.

● Thus the single outer shell electron of sodium would occupy one corner of the cube, while in the case of a noble gas all the eight corners would be occupied.

● This octet of electrons, represents a particularly stable electronic arrangement.

(iii) Lewis postulated that atoms achieve the stable octet when they are linked by chemical bonds.

● In the case of sodium and chlorine, this can happen by the transfer of an electron from sodium to chlorine thereby giving the `Na^+` and `Cl^–` ions.

● In the case of other molecules like `Cl_2`, `H_2`, `F_2`, etc., the bond is formed by the sharing of a pair of electrons between the atoms.

● In the process each atom attains a stable outer octet of electrons.

`color{green}("Lewis Symbols :")` In the formation of a molecule, only the outer shell electrons take part in chemical combination and they are known as valence electrons.

● The inner shell electrons are well protected and are generally not involved in the combination process.

● G.N. Lewis, an American chemist introduced simple notations to represent valence electrons in an atom. These notations are called Lewis symbols.

● For example, the Lewis symbols for the elements of second period are as given in fig.

`=>` The number of dots around the symbol represents the number of valence electrons.

`=>` This number of valence electrons helps to calculate the common or group valence of the element.

`=>` The group valence of the elements is generally either equal to the number of dots in Lewis symbols or `8` minus the number of dots or valence electrons.

`=>` In the periodic table, the highly electronegative halogens and the highly electropositive alkali metals are separated by the noble gases.

`=>` The formation of a negative ion from a halogen atom and a positive ion from an alkali metal atom is associated with the gain and loss of an electron by the respective atoms.

`=>` The negative and positive ions thus formed attain stable noble gas electronic configurations. The noble gases (with the exception of helium which has a duplet of electrons) have a particularly stable outer shell configuration of eight (octet) electrons, `ns^2 np^6`.

`=>` The negative and positive ions are stabilized by electrostatic attraction.

`color{red}("Example :")` The formation of `NaCl` from sodium and chlorine, according to the above scheme, can be explained as :

`underset{ [Ne] 3s^1 }(Na) → underset{ [Ne]}(Na^+) + e^-`

`underset{ [Ne] 3s^2 3p^5} (Cl + e^-) →underset{[Ne] 3s^2 3p^6 , [Ar]}(Cl^-)`

`Na^(+) + Cl^(-) → NaCl ` or `Na^(+) Cl^(-)`

Similarly the formation of `CaF_2` may be shown as :

`underset{[Ar] 4s^2} (Ca) → underset{ [Ar]}(Ca^(2+)) +2e^(-)`

`underset{ [He] 2s^2 2p^5}( F+ e^-) → underset{ [He] 2s^2 2p^6 or [Ne])(F^-)`

`Ca^(2+) +2F^(-) → CaF_2 `or `Ca^(2+) (F^-)_2`
`color{purple}(✓✓)color{purple} " DEFINITION ALERT"`


`=>` The bond formed, as a result of the electrostatic attraction between the positive and negative ions was termed as the `color{red}("electrovalent bond")`.

● The electrovalence is thus equal to the number of unit charge(s) on the ion.

● Thus, calcium is assigned a positive electrovalence of two, while chlorine a negative electrovalence of one.

`=>` Kössel’s postulations provide the basis for the modern concepts regarding ion-formation by electron transfer and the formation of ionic crystalline compounds.

`=>` Kössel and Lewis in 1916 developed an important theory of chemical combination between atoms known as `text(electronic theory of chemical bonding)`.
`color{purple}(✓✓)color{purple} " DEFINITION ALERT"`

`=>` According to this, atoms can combine either by transfer of valence electrons from one atom to another (gaining or losing) or by sharing of valence electrons in order to have an octet in their valence shells. This is known as `color{red}("octet rule")`.