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 Lewis Symbols or Diagrams

Elemental properties and reactions are determined only by electrons in the outer energy levels. Electrons in completely filled energy levels are ignored when considering properties. Simplified Bohr diagrams which only consider electrons in outer energy levels are called Lewis Symbols.
A Lewis Symbol consists of the element symbol surrounded by "dots" to represent the number of electrons in the outer energy level as represented by a Bohr Diagram. The number of electrons in the outer energy level is correlated by simply reading the Group number. Lewis symbols for oxygen, fluorine, and sodium are given in the diagram on the left.

Lewis Symbols for the elements of the second period. Correlate the number of dots with the group number.



A Lewis diagram depicts a mmolecule using an element symbol to represent the nucleus and core electrons of each atom. Valence electrons are represented by lines for electron pair bonds and dots for unbonded electrons.

The following procedure can be followed to derive Lewis diagrams for most molecules.

1. Find the total number of electrons:
Tabulate the total number of outer energy level electrons for all atoms in the molecule. For each atom, read the group number.

2. Draw a first tentative structure:
The element with the least number of atoms is usually the central element. Draw a tentative molecular and electron arrangement attaching other atoms with single bonds as the first guess. Single bonds represented with a line represent 2 electrons

3. Add electrons as dots to get octets around atoms:
When counting electrons for the octet around an atom, count both electrons in a bond for each atom and any lone pair electrons. Hydrogen, of course, gets only 2 electrons.

4. Count the total number of electrons in the final structure to see if the total agrees with the number tabulated in step #1. If not, then move a lone pair of electrons into a double bond. Or add more lone pairs of electrons.

5. Cycle through steps 3 and 4 several times until you get it right by trial and error.