Humboldt State University ® Department of Chemistry

	Fundamentals of Chemistry
	© Humboldt State University

Bonding and Bond Formation

a Quantum Picture

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.

• Notice how spread-out the outer electron (green dot-cloud) of sodium is - it is not very tightly held to the atom.
• On the other hand the outer (green dot-cloud) seven electrons are much more tightly held to the atom - much of the electron density overlaps with the core electrons.
• Finally, in the ionically bonded Na-Cl ion pair note how the outer electron has been stripped from sodium, and the valaence shell has expanded to accomodate the completed shell.

• The movie accessed through the Na + Cl figure shows the formation of a NaCl ion pair in vacuo.
  • Notice how the outer electrons of the sodium atom "jump" to the chlorine atom when they are still well separated.
  • The resulting ions are then attracted to each other until the electron clouds "touch" - interpenetrating slightly and repelling.

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

• You will note in the Cl(2) figure showing the inner, core, electrons of a chlorine molecule (Cl₂) show no overlap. Thus they are not involved in bonding at all, just as you might expect from the Lewis model and Lewis structures.
• The two Cl(2) molecule figure and movies show the overlap of the outer electrons - covalent bonding is a phenomena of the outer, valence electrons.
  • In the middle figure the upper diagram is a plot of the electron density in the x-y plane. There doesn't appear to be much overlap at all of the outer electrons. but keep in mind that only 2 of the 14 outer electrons of the Cl₂ molecule are involved in the bond (and that all of the p electrons are equal and indistinguishable in the filled orbital sets).
  • The lower diagram in this figure shows the corresponding dot-image, while the figure at the right shows the dot-image again in larger size with higher resolution.

 

 

 

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.

 

 

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!

• Notice the gradual overlap which occurs as the atoms approach.
• The Morse curve plots the energy of the system vs. the separation of the nuclei. The stable bond occurs at the low point of the curve. The black portion of the curve shows the very rapidly increasing repulsion as the non-bonding electron clouds begin to overlap.

© R A Paselk

Last modified 4 October 2002