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Theories explaining the covalent bond , Octet rule & Overlapped orbitals concept

From Online Sciences - February 5, 2018

Theories explaining the covalent bond

The concept of the covalent bond developed according to our knowledge about the electron properties , from the theories that are explaining the formation of the covalent bond are : The electronic theory of valency , The valence bond theory and the molecular orbital theory .

The electronic theory of valency ( octet rule 1916 )

This theory was proposed by Lewis and Kosel , where they postulated that the covalent bond is formed when a number of electrons from the outermost shell of the two bonded atoms are shared between nuclei , consequently , the outermost shell of each atom contains eight electrons through the sharing of electron pairs .

Octet rule : With the exception of hydrogen , lithium and beryllium , the atoms of all elements tend to reach the octet structure of the nearest inert gas .

The inadequacies of octet rule

The bonding in many molecules can not be explained on the basis of the octet rule as in :

Phosphorus pentachloride molecule PCl5 , Because the phosphorus atom is surrounded by ten electrons .

15P : [ 10Ne ] , 3s , 3p3

17Cl : [ 10Ne ] , 3s , 3p5

Boron trifluoride molecule BF3 , Because the boron atom is surrounded by only six electrons .

5B : 1s, 3s , 2p1

9F :1s , 2s , 2p5

The simple representation of the covalent bond as being just a shared pair of electrons is not sufficient to explain many of the properties of molecules such as the stereostructure and angles between bonds .

The valence bond theory ( V.B.T )

This theory was based on the conclusions of quantum mechanics that consider the electron not only as a negative particle that move in a definite orbit , but as a material particle with wave property which can exist in any position in the space surrounding the nucleus .

This theory explains the formation of the covalent bond as a result of the overlap of an atomic orbital of one atom which contains a single electron with a similar orbital of another atom , The valence bond theory depends on two main concepts which are : Overlapped orbitals concept & Hybridized orbitals concept .

The valence bond theory : The covalent bond in the molecule is resulted from the overlap of some atomic orbitals in the combined atoms and the rest of the atomic orbitals which didnt take part in the formation of bonds remain as they were in the free atoms .

Overlapped orbitals concept

When two atoms come close to form a covalent bond , an atomic orbital of one atomcontains a single electronoverlaps with a similar orbitalcontains a single electronof another atom .

Application 1 : The formation of hydrogen molecule according to the concept of the overlapped orbitals .

When the two hydrogen atoms come close to each other , an overlapping occurs between 1s orbital ( contains a single electron ) of one atom with another 1s orbital ( contains a single electron ) of the other hydrogen atom to form hydrogen molecule H2 .

Application 2 : The formation of hydrogen fluoride according to the concept of the overlapped orbitals .

When fluorine atom comes close to hydrogen atom , one of the 2pz orbitals of the fluoride atom ( which has a single electron ) overlaps with the 1s orbital ( which has a single electron ) of the hydrogen atom to form hydrogen fluorine molecule HF .

The failure of the overlapped orbitals concept to explain the structure of methane molecule

It is clear from the electron configuration of carbon that the ground state of carbon atom has only two electrons in single state in 2p orbital .

The hybridized orbitals concept

Some of the hybridization type
The explanation of the structure of methane molecule using the hybridized orbitals concept

The molecular orbital theory

The sigma bond ( )
The pi bond ( )

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