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Sigma and pi bonds

October 10, 2012

by Janet G. Coonce MS

 

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Review the notes after viewing the video:

An ‘s’ orbital is in the shape of a sphere.  A ‘p’ orbital is the shape of a dumbbell.  These shapes define the probability of finding an electron in that space.

 

SigmaPiS&P

So in an s orbital the electron can be found anywhere within a sphere surrounding the nucleus as demonstrated on the left.  In the p orbital the electron can be found anywhere above or below the nucleus within a dumbbell shaped orbital illustrated on the right.

 

SigmaPiBonds

A single bond is a sigma bond.

A double bond is a sigma and a pi bond.

A triple bond is a sigma bond and 2 different pi bonds.

 

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If the orbital overlap occurs between the nuclei as illustrated in the drawings on the left it is referred to as a sigma bond.  All single bonds are sigma bonds.  Pi bonds occur when the overlap occurs above and below the nuclei, but the positively charged nuclei are not directly in line with the overlap area.  This arrangement explains why pi bonds are weaker than sigma bonds.  In the illustration on the right, 2 p orbitals overlap to form a pi bond.  On the left, 3 different sigma bonds are demonstrated.  At the top the sigma bond occurs between 2 s orbitals.  In the middle, it is between an s and a p orbital.  This could also represent an sp hybridized orbital.  In the bottom a single sigma bond is illustrated between 2 p orbitals.  In each of the 3 examples of a sigma bond, the electron overlap occurs between the nuclei of the 2 atoms.

 

image

A single bond is a sigma bond.

A double bond is a sigma and a pi bond.

A triple bond is a sigma bond and 2 different pi bonds.

A pi bond is weaker than a sigma bond.

Because a double bond is a sigma and a pi bond, it is stronger than a single bond but not twice as strong.

A triple bond is stronger than a double bond but not 3 times as strong as a single bond.

 

image

In this Lewis dot structure of chloromethane (or methyl chloride) each of the 4 single bonds are sigma bonds.

 

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In this Lewis dot structure of ethylene, each carbon atom has 3 sigma bonds.  One sigma bond is between the two carbon atoms and each carbon atom is connected to 2 atoms of hydrogen by 2 additional sigma bonds.  In addition, there is a pi bond between the two carbons ( a double bond consists of a sigma bond and a pi bond).  Each carbon has electrons forming 3 covalent bonds.  Each electron pair within a bond is trying to get as far away from other bonds.  This arrangement is referred to as sp2 hybridized orbitals.  Each bond is equal distant.  Therefore each the 3 bonds is in the same plane (trigonal planar), and each bond angle is 120 (360/3 = 120).

 

image

In this 3 dimensional model of ethylene, the yellow play dough represents the dumbbell shaped p orbital which will form the pi bond.  The 3 green orbitals represent the sp2 hybridized orbitals.  The carbon nucleus is at the center of the orbitals.   The hydrogen atoms are represented by the white balls which are bonding with the carbon in the sp2 hybridized orbital.

 

image

This illustration shows how the dumbbell shaped p orbitals overlap to form the pi bond and at the same time a sigma bond forms between the green sp2 hybridized orbitals.  All 4 hydrogen atoms are in the same plane.

 

 

Triple Bonding in Ethyne (Acetylene)

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This is the Lewis dot structure of H2C2 (formula of ethyne better known as acetylene).  Between the carbons is a triple bond consisting of 2 pi bonds and one sigma bond.  There are a total of 6 electrons being shared between carbon atoms across the triple bond.  Between the carbon and hydrogen is an sp hybridized sigma bond which is sharing 2 electrons.  The octet rule is satisfied and each carbon has 8 electrons in the hybridized orbitalsAcetylene has planar geometry because each of the bonds will want to be as far apart as possible.

 

 

image

 

The above illustration of the 2 sp hybridized orbitals in each carbon in ethyne was taken from the video “Hybridization Geometries & Bond Angles.”  To form the triple bond in ethyne a sigma bond forms between one of the sp hybridized orbitals of each carbon.  Hydrogen forms a sigma bond between the other sp hybridized orbital of each carbon.  The 2 dumbbell shaped p orbitals each form a pi bond.  A triple bond is 2 pi bonds and one sigma bond.

 

 

 

 

 

Transcription by James C. Gray MD FACOG

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