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Molecular orbital theory & Valence shell electron pair repulsion theory

From Online Sciences - February 6, 2018

Sigma bond is stronger than pi bond , Because sigma bond is formed from the overlapping between the hybrid orbitals together or with the atomic orbitals by head , while pi bond is formed from the overlapping of the atomic orbitals only by side .

The explanation of the structure of ethylene molecule using the molecular orbital theory

The excitation process takes place for the two carbon atoms by acquiring the required amount of energy , After the two carbon atoms have been excited , the three orbitals of 2s , 2px and 2py are hybridized forming three hybridized orbitals , each of them is named sp and contains one single electron .

To overcome the repulsive forces between the orbitals , they are directed in space as far apart as possible , thus the angles between them are 120 , The atom then will be more stable and the molecule takes the planar triangle shape .

It is observed that the 2pz orbital of each carbon atom is not involved in the hybridization process , These two orbitals lie perpendicular on the plane of the three sp hybrid orbitals of each carbon atom , There are two types of the overlapping between carbon atoms and two types of bonds are formed , which are :

The sigma bond ( ) which is formed as a result of the overlap by head to head between : Twosp hybrid orbitals of each carbon atom with two 1s orbitals of the two hydrogen atoms , forming the CH bonds , The thirdsp orbital of one carbon atom with the similar one of the other carbon atom , forming the CC bond .

The pi bond ( ) which is formed as a result of the overlap side by side between the two2pz orbitals of the two carbon atoms , forming theCC bond between the two carbon atoms .

The explanation of the structure of acetylene molecule using the molecular orbital theory

It is clear that after the excitation process has occurred in the two carbon atoms by acquiring the required amount of energy , hybridization occurs between one orbital 2px from the sub-level ( p ) orbitals and the 2s orbital producing two sp hybridized orbitals , each one of them contains one single electron .

To overcome the repulsive forces between the two hybridized orbitals , they are directed in space as far apart as possible , forming an angle of 180 , The atom then will be more stable and the molecule takes a linear shape .

It is observed that in each carbon atom , there are two atomic orbitals remaining , such as2py and 2pz are not involved in the hybridization process , The two carbon atoms are bonded together and with hydrogen atoms by two types of bonds as the following :

The sigma bond ( ) produced from the head to head overlap between :

The pi bond () produced from the side by side overlap between :

The valence shell electron pair repulsion ( VSEPR ) theory

The shapes of molecules differ according to ( free and bonded ) electron pairs , which are found in the orbitals of the central atom in the covalent molecule .

Shapes of the covalent molecules according to VSEPR theory

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