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Lewis Symbols

Lewis Symbols are simplified Bohr diagrams which only display electrons in the outermost energy level. The omitted electrons are those in filled energy levels, which do not contribute to the chemical properties of the species in question. Therefore Lewis Symbols are useful for studying elemental properties and reactions.

Introduction

A Lewis Symbol is constructed by placing dots representing electrons in the outer energy around the symbol for the element. For many common elements, the number of dots corresponds to the element's group number.  Below are Lewis Symbols for various elements. Notice the correspondence to each element's group number.

201lewis.GIF

201lewis2.GIF

Lewis Diagrams for Molecules

Molecules can be depicted by Lewis Diagrams by placing dots or lines around the constituent elemental symbols. Once again only valence electrons are shown. Lines denote bonded electron pairs, whereas dots are reserved for unbounded electrons. The following algorithm can be used to construct Lewis diagrams of most molecules.

  1. Find the total number of electrons: Determine the total number of valence electrons by reading the group number for each element.
  2. Draw a first tentative structure: Frequently the central element is the one with least atoms in the molecule. In your first draft, attach the atoms with single bonds.
  3. Add electrons as dots to get octets around atoms: Each atom must have its valence shell completely filled. Remember to count bonds and lone electrons.
  4. Count the total number of electrons: Ensure the number of electrons displayed in the symbol agree with the calculation from step 1.
  5. Cycle through steps 3 and 4: By way of trial and error, repeat the steps above until a conclusion is reached.

201lewis3.GIF

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