Formulas and Diagrams

Let's look at how molecular formulas, electron dot diagrams and structural formulas are related to one another for several simple compounds. The molecular formulas show the correct number of each type of atom in the molecule. The electron dot diagrams (Lewis structures) show the arrangement of those atoms and all of the valence electrons. The structural formulas show the arrangement of the atoms and the covalent bonds between them. The following diagrams are also shown in Example 2 in you workbook. Note that the structures shown here are slightly different from what is in your workbook. This is because all of these diagrams are left-right-up-down representations of the actual three-dimensional molecules and as such can be shown with different orientations. Not only is there random variation in how these formulas are drawn, the medium (chalkboard, word processing, HTML) dictates which orientations are easiest to format.

Molecular Formula	Electron Dot Diagram	Structrual Formula
CH₄	H ЗЗ H : C : H ЗЗ H	H \| H - C - H \| H

First we have the molecular formula CH₄. In the electron dot diagram you can see how the four valence electrons of carbon match up with the one valence electron of each of the four hydrogen atoms to give eight electrons around carbon. The electrons are arranged in four pairs representing four covalent bonds. You should remember that the arrangement of those four pairs of electrons is called a tetrahedral arrangement. The structural formula emphasizes the bonds rather than the electrons and shows one line for each of the bonds that connect the atoms together. Each line represents one shared pair of electrons.

Molecular Formula	Electron Dot Diagram	Structrual Formula
CH₂O	: O : :: H : C : H	O \|\| H - C - H

Next you can see the same things for CH₂O. However, in this case, the resulting structural formula is different in two ways. First, since there are two pairs of electrons shared between the oxygen atom and the carbon atom, a double line is drawn. Second, since the other two pairs of electrons around the oxygen atom do not form a bond to another atom, they are not shown in the structural formula.

Sometimes, when trying to emphasize the effect of unbonded electron pairs on structure or bonding or Lewis base properties, the unbonded electron pairs will be shown in structural formula. Usually, however, they will be left out.

You should also remember that the arrangement of three groups of electrons around this carbon atom is called trigonal planar or flat triangular arrangement. Sometimes to emphasize that, the structural formula is drawn this way.

H
\
      C = O
/
H

Molecular Formula	Electron Dot Diagram	Structural Formula
HCN	H : C ::: N :	H - C К N

With HCN we have a triple bond between the carbon and nitrogen. The electrons around the central carbon atom are clustered into two groups, the single bond to hydrogen and the triple bond to nitrogen. You should remember that this is called a linear arrangement and results in a linear molecule.

Later (in lesson 2) we will take a closer look at the arrangement of electrons in molecules and you will learn that these electron dot diagrams are overly simplistic. Nevertheless, they are valuable and useful. Perhaps this is because they are simplistic.

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E-mail instructor: Eden Francis

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