Electronic Devices and Circuits Engineering Sciences 154

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 click on images

Carbon-Hydrogen Covalent Bonding
click on image	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.

click on images

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