Valence Bond Model of the Double Bond
(ethylene as the example)
At left (or above on narrow screen) is a 3-D version of the Lewis structure of the ethylene
molecule. You may rotate the molecule yourself by holding the
mouse button down while dragging it around within the image
frame. Dark gray spheres represent C atoms and light gray
(white) spheres represent hydrogen atoms. Single bonds are
represented as single sticks and double bonds as a pair of
sticks. To return to this view at any time click on the button
below:
Each carbon atom is surrounded by three "groups" as specified in the VSEPR
model for molecular shapes. An atom surrounded by three
groups will be sp2
hybridized to get orbitals pointing in the correct directions to
make bonds at 120° from each other. Click the button below to
show these hybrid orbitals on one C.
Single bonds (σ-bonds) are made by overlapping hybrid orbitals pointed
along the bonds. The σ-bond that is part of the double bond in ethylene
is made by the overlapping sp2 hybrids on the neighboring C
atoms. Click on the button below to show the hybrid orbitals on the
other C atom and the overlap forming the σ-bond.
The bonds to the H atoms are made by overlapping the remaining hybrid
orbitals with the H 1s orbitals. Click on the button below to add
the H 1s orbitals and thus show the whole σ-bond network.
π-bonds are made by the side-on overlap of p orbitals on neighboring
atoms. To add the two p orbitals that are left over on the
carbons from the formation of the sp2 hybrid orbitals click
on the button below.
The yellow orbitals are the p orbitals.
To show how the two p orbitals combine to make a π-bond click on the
button below.
The red surface indicates how the two yellow p orbitals overlap. To
remove the p orbitals and leave the π-bond click on the button below.
This model shows the double bond as the σ-type overlap of the sp2
hybrid orbitals (in blue) plus the π-type overlap of the p orbitals (in
red). The electron density above and below the bond in the π-bond
prevents the two ends of the molecule from rotating relative to each
other about the C–C double bond; thus maintaining the planar geometry of the
molecule.
To show just the π-bond click on the button below.
You may look at any of these intermediate views again by clicking on
the appropriate button.
Viewer Instructions
You can interact with the image. It is displayed using JSmol.
Rotate the image
Hold down the mouse button and drag the cursor over the
image.
Zoom the view
Hold down the shift key and the mouse button while
dragging up or down over the image. On a touchpad two finger drag-up
zooms in.
Many more options in the right-click menu
The right-click menu can also be activated by
clicking on the JSmol icon at the lower right of the
viewer.
The blue s and sp2 orbitals plus the yellow p orbital are the standard
JSmol/Jmol
cartoon representations for these types of orbitals.
The red π orbital is generated from the results of a calculation done with
the GAMESS1ab initio quantum mechanics
package to find the optimal geometry of ethylene using the 3-21G basis set. The compute resources were
provided by UW Oshkosh.