Ethene electron density distribution

Description of the electronic stmcture of molecules by using this molecular orbital model provides important information about the nature of molecules. One of the most important pieces of information is the electron density distribution. While in a -bonds the electron density is largest between the atoms i.e. along the line of the chemical bond in % bonds the electron density is concentrated not between the atoms but above and below the plane which contains the bond line . This is represented in the next figure, hi an ethane molecule, which possess only a -bonds, the electron cloud is situated between the C-atoms. However, in the ethene molecule, with a x-bond, the electron density is highest above and below the plane in which all the atoms are located. Electron clouds in ethene are not blocked by the atoms and are situated on the open side of the molecule where other particles can attack it. Consequently an ethene molecule is much more chemically reactive then ethane. 

The electron density distribution of both the a and 7t bonds in ethene is shown in Figure 4.27. 

Ethene is a planar molecule because this ensures the maximum overlap of the p orbitals that form the n bond. You will notice that the electron clouds that make up the 71 bond lie above and below the plane of the carbon and hydrogen nuclei. We would expect the H—C—H bond angle in ethene to be about 120° because the three areas of electron density of the a bonds are equally distributed. However, because of the position of the n bond, this bond angle is actually 117°. This minimises the repulsive forces. 

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