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The following procedures are a systematic approach to preparing
a correct Lewis Dot Structure for a covalently bonded molecule.
Each of these steps will be further discussed on individual pages
in this presentation.
- Determine the total number of valence
electrons in the system.
- Determine the central atom. Most
basic structures will be built around a single central atom.
The central atom will usually be the element with the lowest
electronegativity, or the element that appears as a single atom
in the formula. Arrange the
external atoms in a symmetrical fashion around the central atom.
Since three-dimensional geometry is not being determined, it
is not critical to worry about a specific placement of the external
atoms. Use an arrangement that is symmetrical and easy to work
with.
- Using a single pair of electrons
for each bond, bond each external atom to the central atom.
Place additional pairs of electrons
around the external atoms, in order to achieve the Octet Rule.
Hydrogen atoms will be the most common exception to the Octet
Rule, in that they will not surround themselves with 8 electrons,
but with only two electrons, or one pair.
- Place remaining additional pairs
of electrons around the central atom, up to a maximum of a total
of 8 electrons located on the central atom, or until the supply
of electrons has been exhausted.
- Check to make sure that each atom
has fulfilled the Octet Rule, with the exception of Hydrogen.
If the central atom has less than 8 electrons around it, then
bring in one, or more, nonbonded electron pairs from an external
atom and place them as an additional bond between that external
atom and the central atom. This will provide additional pairs
of electron for the central atom, yet, will not be taking electron
pairs away from the external atoms. Continue this process until
each atom has a total of 8 electrons around it. (Exception: Hydrogen)
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