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Rules for Writing Lewis Dot Structures - General Chemistry I | CHEM 151, Study notes of Chemistry

lewis dot rules Material Type: Notes; Class: General Chemistry Lecture I; Subject: Chemistry; University: Lansing Community College;

Typology: Study notes

2011/2012

Uploaded on 03/04/2012

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Rules for Writing Lewis Dot Structures:
1. Write the bonding sequence:
a) For simple molecules and polyatomic ions, the first atom in
the formula is usually the central atom and all other atoms
are bonded to it.
b) Hydrogen is always a terminal atom.
c) In general, molecules with symmetrical formulas tend to
have symmetrical structures. For example S2Br2 has the
following skeleton structure: Br-S-S-Br. Note that the less
negative sulfur atoms follow rule 1a.
d) The oxy-acids are an exception to rule 1a. The oxygen
atoms of oxyacids are bonded to the central atom and the
hydrogen atoms are bonded to the oxygen atoms. For
example, chlorosulfonic acid (HSO3Cl) has the following
skeleton:
O
S
Cl
OO H
e) When several possible skeletons exist, choose the one in
which the atoms have their usual number of covalent
bonds.
2. Add up the total number of valence electrons in the molecule.
Do this by adding together the valence electrons for each
atom. Do not forget to add or subtract electrons for anions or
cations.
3. Subtract from this total the number of electrons used to form
single bonds in rule 1. Remember each single bond counts as
two electrons.
4. Complete octets of all atoms using nonbonding electron pairs.
Subtract this number of electrons from those not used to form
the single bonds.
.
5. If there are not enough electrons for rule 4, consider multiple
bonds. (C, N, O, and S often form multiple bonds.) You will
need one multiple bond for every two electrons you are short
in rule 4.
6. If there are too many electrons for rule 4, assign the extra
electrons to the central atom.
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Rules for Writing Lewis Dot Structures:

  1. Write the bonding sequence: a) For simple molecules and polyatomic ions, the first atom in the formula is usually the central atom and all other atoms are bonded to it. b) Hydrogen is always a terminal atom. c) In general, molecules with symmetrical formulas tend to have symmetrical structures. For example S 2 Br 2 has the following skeleton structure: Br-S-S-Br. Note that the less negative sulfur atoms follow rule 1a. d) The oxy-acids are an exception to rule 1a. The oxygen atoms of oxyacids are bonded to the central atom and the hydrogen atoms are bonded to the oxygen atoms. For example, chlorosulfonic acid (HSO 3 Cl) has the following skeleton: O S Cl

O O H

e) When several possible skeletons exist, choose the one in which the atoms have their usual number of covalent bonds.

  1. Add up the total number of valence electrons in the molecule. Do this by adding together the valence electrons for each atom. Do not forget to add or subtract electrons for anions or cations.
  2. Subtract from this total the number of electrons used to form single bonds in rule 1. Remember each single bond counts as two electrons.
  3. Complete octets of all atoms using nonbonding electron pairs. Subtract this number of electrons from those not used to form the single bonds. .
  4. If there are not enough electrons for rule 4, consider multiple bonds. (C, N, O, and S often form multiple bonds.) You will need one multiple bond for every two electrons you are short in rule 4.
  5. If there are too many electrons for rule 4, assign the extra electrons to the central atom.
  1. Check to make sure the number of electrons in your Lewis dot structure matches the total number available from rule 2.