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The Lewis Electron-Dot Formulas

Lewis Electron-Dot Formulas: Lewis electron-dot formulas are diagrammatic representations of the atoms involved and their valence electrons. The valence electrons are usually represented as dots around the elemental symbol. It is a two-dimensional way of representing the structural formula, showing the bonding electrons and the lone electrons that are in the valence shells.

Writing Lewis Formulas

The objective of this section is to become comfortable with writing Lewis structural formulas. We can only predict the Lewis structures of simple molecules. Other complex structures require complicated analysis and predictions based on experimental data.
  1. First, determine the main structural make up of the molecule, such as guessing which atom will be the central atom of the molecule. The central atom of the molecule is usually the atom with the lowest electronegativity.
  2. Next, determine the total number of valence (outermost) electrons.
  3. Draw the basic skeletal structure of the molecule or ion.
  4. Next, determine the distribution of those valence electrons so as to complete the octet of the atoms that are around or bonded to the central atom.
  5. The remaining electrons are to be distributed in pairs around the central atom.
  6. Sometimes you might find that the central atom is not reaching the octet level even at this point. The most likely reason is that there might be a need of a double bond or a triple bond.
Let's go through an example. Before looking at the solution of the example shown below, determine the Lewis structure on your own.

Example 5-1

 Write the Lewis dot formula of carbonate ion



Since carbon is the least electronegative, it is the most likely atom to be in the center. With this information, we can draw the carbonate ion as indicated below.


image\29365 ch 5 page 67.png


Next, calculate the number of valence electrons. Carbon has 4 electrons, whereas oxygen has 6 electrons each, and do not forget the net charge of –2, which accounts for two more electrons. This tallies to a total of 24 electrons. We can set up all these valence electrons around individual atoms in the structure drawn so far, as shown below:


chemistry-web-resources\Experimental Graphics\24531PV.jpg


Now that we have set all the valence electrons, we are done, right? No! Watch out for the carbon. The octet of carbon is not satisfied, and thus carbon will not be very stable with its valency at this point. Since there are no more electrons to spare, we have to move one of the pairs of electrons from an oxygen atom, which results in 1 double bond and 2 single bonds. The completed Lewis structure of the carbonate ion should look as follows:


chemistry-web-resources\Experimental Graphics\24539PV.jpg

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