LEWIS ELECTRON–DOT STRUCTURES

Lewis structures will be our first step in describing several models which will help us organize a large amount of experimental data. We can apply our knowledge of the electronegativity and the octet rule to extent our ability to write more complicated Lewis electron–dot formulas. Practice writing Lewis formulas will help us understand many of the ratios of elements, bonding geometry and polarity and intermolecular attractive forces that occur in covalent compounds. The object will be to predict a reasonable Lewis electron–dot structure given the chemical formula. To do this we must follow some rules which are stated in your text (Page 269).

1. Write the skeletal structure for the molecule.

   • most electronegative atoms are terminal (O, F, Cl)

   • molecules with symmetrical formulas may have symmetrical structures

2. Determine the total number of valence electrons by adding the valence electrons for all atoms. For anions, add an amount equal to the charge of the anion to the total number of valence electrons. For cations, subtract an amount equal to the charge of the cation from the total number of valence electrons.

3. Count up the electrons required to build the skeletal structure in part 1), and subtract from the number of electrons in part 2).

4. Distribute the remaining electrons so each terminal atom atom has an octet of electrons. Any remaining electrons go to the central atom. Note: it maybe necessary to involve multiple bonds.

We will discuss some examples to demonstrate the procedure for writing correct Lewis electron–dot structures.

Example #1

Example #2

Example #3;

Example #4;

Multiple bonds are formed when two atoms in a chemical bond share more than one pair of electrons. In the example above 2 pairs of electrons were placed between the carbon and the oxygen. This is referred to as a double bond. A double bond is stronger than a single bond. The Lewis structure of the molecule CO demonstrates another type of multiple bond. The correct Lewis structure for carbon monoxide has three pairs of electrons between the carbon and oxygen. Such an arrangement is called a triple bond. This is stronger than a carbon oxygen double bond.

The C–O bond distance in the series CH₃OH (C–O is 1.43 Å), Cl₂CO (C–O is 1.22 Å) and CO (C–O is 1.13 Å). The more electrons between the atoms the short the distance between the atoms and the stronger the bond.

Sample Problem #5;