Problem Solving

Number of Bonds: The Pauling-Giddings Rule-An Easy, Fast Way to Determine Lewis-dot Structures

Determining electron dot structures for molecules is relatively easy if one knows how many bonds to draw. A powerful little bonding rule is

The following examples show how easy this is to use.

Example 1: How many bonds are present in a N2 molecule?

Step 1: Each N atom "demands" an octet (8) of electrons, so D = 2 x 8 = 16.

Step 2: Each N atom "supplies" the five valence electrons it has, so S = 2 x 5 = 10.

Step 3: B = (D - S)/2 = (16 - 10)/2 = 3; there are three bonds-a triple bond. Draw the triple bond between the N atoms, then complete the octet around each N atom until all 10 "supplied electrons" are used.

Example 2: How many bonds are present in a carbonate ion, CO32- ?

Step 1: The C atom "demands" an octet (8) of electrons and each O atom "demands" an octet, so D = (1 x 8) + (3 x 8) = 32.

Step 2: The C atom "supplies" the four valence electrons it has, and each O atom "supplies" six valence electrons. However, we also must consider the net ionic charge (see next step).

Step 3: Electrons have been supplied due to the 2- charge on the ion, so S = (1 x 4) + (3 x 6) + 2 = 24.

Step 4: B = (D - S)/2 = (32 - 24)/2 = 4; there are four bonds between the three oxygen and one carbon atom (from an electron-dot viewpoint, one can be regarded as a double bond; two as single bonds).

Step 5: Draw the double bond between C and one of the O atoms, then two single bonds between the C and the other two O atoms. Finally, complete the octet around each atom until all 24 "supplied electrons" are used. (NOTE: There are three different ways to draw the C = O bond, corresponding to the three resonance structures for this ion.)

Bonding "Coverage"

What is the minimum that should be included when teaching bonding in an introductory chemistry course? One list of minimum bonding concepts might look like this:

  1. Covalent bonds and covalent network solids

  2. Ionic bonds

  3. Using electronegativity

  4. Metallic bonds

  5. van der Waals forces

  6. Polar covalency

  7. Lewis-dot formulas for molecules and ions

  8. Valence electrons and the periodic table

Other bonding topics can be regarded as optional even thought they may appear in the textbook. For example, concepts such as hybrid bonds are not absolutely essential.
Chemical Bonding (BOND)
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