Notes Lewis Dot Structures
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Rules for Writing the Skeleton Structure of a Molecule
1. Many small molecules or polyatomic ions consist of a central atom around which are
bonded atoms having greater electronegativity, such as F, Cl, and O. In some cases, H
atoms surround a less electronegative atom because H atoms can form only one bond.
For example, CCl4 and CH4:
Cl

Cl  C  Cl

Cl
H

H CH

H
2. Molecules or polyatomic ions that have symmetrical formulas usually have symmetrical
structures. For example, disulfur dichloride:
Cl  S  S  Cl
3. Oxoacids are substances in which O atoms (and possibly other electronegative atoms) are
bonded to a central atom with one or more H atoms usuallybonded to O atoms. For
example, perchloric acid, HClO3:
O

O  Cl  O  H

O
4. Of several possible structures, the one in which the atoms have their usual number of
covalent bonds is usually preferred. For example, oxygen difluoride, OF2:
FO

F
Steps in Writing Lewis Dot Structures
1. Calculate the total number of valence electrons for the molecule by adding up the
number of valence electrons for each atom. If we are writing the Lewis formula for a
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negatively charged polyatomic ion, we ADD the number of negative charges to this total
(because there are more electrons than are provided by a neutral molecule). If we are
writing the Lewis formula for a positively charged polyatomic ion, we SUBRACT the
number of positive charges from the total (because there are fewer electrons than are
provided by a neutral molecule). (Please remember that plain numbers do not have
a + or - sign! “Number of...” means just that: the NUMBER!!))
2. Write the skeleton structure of the molecule or ion, connecting every bonded pair of
atoms by a pair of dots (or a dash).
3. Distribute electrons to the atoms surrounding the central atom (or atoms) to satisfy
the octet rule (having 8 valence electrons) for these atoms (Remember that hydrogen can
only make one single covalent bond.)
4. Distribute the remaining electrons as pairs to the central atom (or atoms), after
subtracting the number of electrons already distributed from the total of valence electrons
calculated in Step 1.
If there are fewer than 8 electrons on the central atom, this suggests a multiple bond
is present (double or triple).
Two electrons fewer than 8 suggests a double bond; four fewer suggests a triple bond or
two double bonds.
To write a multiple bond, move one or two electron pairs (depending on whether the
bond is a double or triple bond) from a surrounding atom to the bond connecting the
atom. Atoms that often form multiple bonds are C, N, O, P, and S.
Rules for Determining Exceptions to the Octet Rule:
1. NOT ENOUGH ELECTRONS: BeH2
2. ODD NUMBER OF ELECTRONS: NO
3. THIRD, ETC. PERIOD ELEMENTS: PCl5; SF6
THESE ALSO USE d ORBITALS
OCTET RULE IS BASED ON ONE s AND THREE p ORBITALS
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