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chemical bonds
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The union of two or more atoms involving redistribution of electrons either by transfer or sharing between themselves, so that all of them acquire the stable noble gas configuration of minimum energy is known as chemical bonding.

The attraction between atoms within a molecule is called a chemical bond.

Electronic theory of valency :

The electronic theory leads to the following types of combination between atoms.

1) Ionic or Electrovalent Bond :
Involving transfer of one or more electrons from one atom to another.
2) Covalent Bond :
Mutual sharing of electrons between two atoms.

Octet rule : Formation of many ionic and covalent compounds can be explained on the basis of the octet rule which states that "Atoms of elements tend to acquire an octet of electrons (i.e. eight electrons) in their outermost shell through combination with other atoms." In doing so they tend to achieve electronic configuration of the nearest inert gas.

However there are some limitations with the octet rule as it fails to explain incomplete and expanded octets.

The Ionic Bond

An ionic bond is formed by transfer of electrons from one atom to another atom in the molecule.

 Consider the formation of potassium chloride. An electron is transferred from potassium atom to chlorine atom. The resulting K+ and Cl- ions possessing configuration of argon (2,8,8) combine to form an electrovalent compound as shown below :

 Another example:

The total number of electrons lost by Lithium must equal the total number of electrons gained by Bromine. Thus the number of Lithium ions produced is the same as the number of bromide ions produced.

 When positive ions (cations) and negative ions (anions) come closer to each other, they are held by electrostatic forces of attraction.

The electrostatic attraction between the oppositely charged ions is called the ionic bond.

Covalent Bond

A Covalent Bond is formed due to sharing of electrons between two atoms.

When both the atoms ( similar or dissimilar ) taking part in a chemical combination are short of electrons to complete the nearest inert gas configuration, the combination between them takes place by sharing electrons. Each atom contributes one electron to form a common pair which, then is shared by both.

The Bond established between atoms by this process of sharing is known as a covalent bond.

A covalent linkage is expressed by a dash (-) in writing formula. This system of representation of formula of a substance is known as electron dot (lewis) and dash formula.

Other examples of covalent compounds where dissimilar atoms combine are, NH3 and CH4.