Electronic Devices and Circuits
Engineering Sciences 154

Covalent Bonds

Consideration of the electronic structure of atoms - particularly the stability of the nobel gas configurations - leads us directly to the notion that atoms will bond together to reach an octet.

From Auburn University's Molecular Structure
 

 

From Access Excellence's Covalent and Ionic Bonds
The Ionic Bond: Ionic bonds are formed when there is a complete transfer of electrons from one atom to another, resulting in two ions, one positively charged and the other negatively charged. For example, when a sodium atom (Na) donates the one electron in its outer valence shell to a chlorine (Cl) atom, which needs one electron to fill its outer valence shell, NaCl (table salt) results.  Ionic bonds are often 4-7 kcal/mol in strength. (source)

The Covalent Bond: Covalent Bonds are the strongest chemical bonds, and are formed by the sharing of a pair of electrons. The energy of a typical single covalent bond is ~80 kilocalories per mole (kcal/mol).  However, this bond energy can vary from ~50 kcal/mol to ~110 kcal/mol depending on the elements involved. Once formed, covalent bonds rarely break spontaneously. This is due to simple energetic considerations; the thermal energy of a molecule at room temperature (298 K) is only ~0.6 kcal/mol, much lower than the energy required to break a covalent bond. (source)

 
Carbon-Carbon Covalent Bonding

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Carbon-Hydrogen Covalent Bonding
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Bond Number
Example
Energy
(kcal/mol)
single
H
|
H--C--H
|
H
~80
double
H  H
|  |
H--C==C--H
|  |
H  H
~150
triple
H
|
C
|||
C
|
H
~200
Hydrogen-Oxygen Bonding
Covalent bonds can also have partial charges when the atoms involved have different electronegativities. Water is perhaps the most obvious example of a molecule with partial charges. 
The symbols delta+ and delta- are used to indicate partial charges.   Oxygen, because of its high electronegativity, attracts the electrons away from the hydrogen atoms, resulting in a partial negative charge on the oxygen and a partial positive charge on each of the hydrogens.  The possibility of hydrogen bonds (H-bonds) is a consequence of partial charges. 
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The Covalent Bond in Three Dimensions - Crystal structure of the diamond covalent structure:
Interactive diamond and zincblende structures ( GaAs, Si, etc.The "Crystal Viewer" - unitcells with lists of material properties.  The applet shows a 3D view of the unitcell and a 3D view of the covalent bonds.  The two figures below are taken from this very valuable applet
Further details on the tetrahedral covalent bonds in the diamond structure

This page was prepared and is maintained by R. Victor Jones
Comments to: jones@deas.harvard.edu.

Last updated October 14,  2001