Chemistry Revision of Chemical Bonding For NDA part-1
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Chemical Bond

Chemical Bond may be defined as a force that acts between two or more atoms to hold them together as a stable molecule. During the formation of a chemical bond, energy is released.

Electronic Theory of Chemical Bonding (Octet Rule)

Kossel and Lewis in 1916 developed a theory of chemical combination between atoms known as electronic theory of chemical bonding based on inertness of noble gases.

Acording to this theory "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 or stable configuration just like noble gases in their valence shells". This attainment of eight electrons for stable structure is called the octet rule.

Valency

`=>` The capacity of an element to form chemical bonds is known as its valency. The valency of an element is the combining capacity of an element.

`=>` The valency of an element decides the number of other atoms which can combine with one atom of that element. e.g. the valency of carbon is 4 and that of hydrogen is 1. So, one atom of carbon can combine with four atoms of hydrogen to form a methane molecule `CH_4` .

`=>` Valency of metal = Number of valence electrons in its atom. e.g. sodium (Na) = 2, 8, 1

`:.` Valency of sodium = 1

`=>` Valency of non-metal = 8- Number of valence electrons

in its atom. e.g. chlorine `Cl_17` = 2, 8, 7

`:. ` Valency of` Cl = 8-7 =1`

`=>` Valency of non-metal = 8- Number of valence electrons in its atom. e.g. chlorine `Cl_17 = 2, 8, 7`

`:.` Valency of `Cl = 8-7 =1`

Valencies of some important elements are as follows :

(i) H, Cl, Br, I, Cu (ous) etc. are monovalent as their valency is 1 .

(ii) O, S, Mg, Ca Zn, Fe (ous), Cu (ic), Pb, Co, Ni etc.

are divalent as their valency is 2.

(iii) N, P, Al, Cr, B etc are trivalent as their valency is 3.

(iv) C, Si etc are tetravalent as their valency is 4.

Some elements e.g., Cu, Fe etc. exhibit more than one valencies (variable valency). Cu and Hg exhibit +1 and +2 valencies and Fe exhibits +2 and +3 valencies.

 
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