Humboldt State University ® Department of Chemistry

Richard A. Paselk

 

Bonding and Bond Formation

a Quantum Picture*

Richard A. Paselk

Note to the User: all of the animations are set to go through one cycle of rotation or bond formation. After an initial viewing you may want to click on the scrollbar and arrow-keys to its right to "step-through" the animations.

Review terms: valence shell, electropositive and electronegative, ionic and covalent bonds, molecule, Lewis structure, non-bonding & lone-pair electrons.

Ionic bonds are formed when one or more electrons are transferred from one atom to another, with the resulting ions held together by electrostatic forces. Note that these are strong, but they are non-specific and can easily "transfer" from one ion to another, so they tend to be unstable. Sodium chloride is an excellent and clear-cut example of ionic bonding . To help understand this system images and movies are provided for sodium and chlorine atoms and an ion pair. Note that the inner, "core" electrons for both atoms are shown as yellow dots, while the valence electrons for both atoms are shown as green.

 

 

 

Covalent bonds are formed when we have a sharing of electrons.

     

 
 
Covalent bond formation
As we saw in the QuickTime movies above, covalent bonds are formed when two atoms share one or more electron pairs - there is an overlap of the orbitals of the two atoms. In the simplest case, that of hydrogen, the resulting bond and molecule are cylindrically symmetrical, as seen in the figure and QuickTime movie of hydrogen. You might also note that hydrogen is nearly spherical as a molecule because the nuclei can approach each other so closely since there is no inner electron shell. Cylindrically symmetrical bonds like hydrogen's are known as sigma bonds. They may be formed by overlap of two s orbitals as in hydrogen, an s orbital and a p orbital lobe, two p orbital lobes (as seen in Cl2 above) etc.
   
Another view of the bonding process is shown in the false colors below, where the electron density is indicated by a rainbow pallet with red symbolizing the highest density and violet the lowest density:
 
As seen in the Morse curve below the two hydrogen atoms come together until the energy is minimized. The H2 bonding QuickTime movie visualizes this process, the movement of the atoms corresponding to the colored region of the Morse curve.

Finally, the bonding movie for chlorine is shown below along with its Morse curve. The green region of the curve corresponds to the movie. If you are off campus note the large download size of the movie!

   
As noted above for the formation of hydrogen molecules, the energy of bond formation appears as vibration and/or rotation of the new molecule around its center of mass.

*The animations and visualizations on these pages are copyrighted. They were created by Mervin P. Hanson, Richard L. Harper, Richard A. Paselk and John B. Russell from calculations performed by Mervin P. Hanson. This work was supported by the National Science Foundation, Apple Computer, and Humboldt State University.


© R A Paselk

Last modified 1 May 2007