Humboldt State University ® Department of Chemistry

Richard A. Paselk

Atomic Structure and Bonding - A Quantum View

© R. Paselk 2004

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Structure and Bonding* Supplement

Atomic Structure: a short review

Atomic structure is characterized/determined by the Atomic number, Z, which tells us how many electrons and protons the free atom has.

Electrons around atoms are arranged in shells : regions of electron occupancy having the same average radial distance from the nucleus. As additional electrons are added additional shells are added making the atom larger. (Note that each additional shell shows up as a new period in the Periodic table.)


Electronic Configurations of Atoms: Under normal earth conditions atoms are in their ground state configurations, that is the electrons all occupy the lowest energy orbitals available. Of course only two electrons of paired spin may occupy an orbital. And electrons "spread out" to occupy as many orbitals in each subshell (orbital type) as possible.



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



Bond Formation and Bonding - the Quantum Picture
As we saw 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.
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 image below gives a cross-section of the electron density for the hydrogen bonding process. In this image, the density is shown as a false color map, with violet indicating low density, and going to red for high densty.


*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 23 November 2004