Drawing Lewis Structures Writing Lewis Dot Structures

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Drawing Lewis Structures Writing Lewis Dot
Structures
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Step 1 Know how to determine the valence
electron for all elements.
Definition Valence Electrons An electron in
an outer shell of an atom that can participate in
forming chemical bonds with other atoms.
Example H has 1 valence electron F has 8 valence
electrons Carbon has 4 valence electrons
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Drawing Lewis Diagrams for Molecules

• Step 1 Draw each element separately, showing the
  valence electrons. (Use different symbols for
  different atoms)
• Step 2 Choose the least occurring atom as the
  central atom (or the least electronegative).
  Except hydrogens or Oxygens.
• Step 3 Add next frequently occurring atoms around
  the central atom keeping symmetry in mind.
• You cannot put two of one atom next to each other
• Ie/ OOAl must be O Al O
• Step 4 Add all the electrons and then put
  Hydrogen on LAST, oxygen atoms on 2nd to last.

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Drawing Lewis Diagrams for Molecules
Step 5 Never leave an unpaired electron Step 6
Circle your 8s starting with the central
atom Group 1 2e- H Group 2 2e- or 4e-
Mg Mg
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Step 2(Connectivity) From the Chemical Formula,
determine the atom connectivity for the
structure.Find the central atom.

• Examples
• i. Given a chemical formula, ABn, A is the
  central atom and B flanks the A atom.
• i.e., NH3, NCl3, NO2. In these examples, N
  is central in the structure.
• ii. H and F are never central atoms.

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Valence electrons and number of bonds Step 3
( of Bond) Determine the number of bonds in the
compound

• Recall the number of bonds at atom prefers
  depending on the number of valence electrons

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In general, these are the number of bonds formed
by these atoms.
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Lewis Structure, Octet Rule Guidelines

• When compounds are formed they tend to follow the
  Octet Rule.
• Octet Rule Atoms will share e- until it is
  surrounded by eight valence electrons.
• Rules of the game-
• i) O.R. works mostly for second period elements.
• Many exceptions especially with 3rd period
  elements (d-orbitals)
• ii) H prefers 2 e- (electron deficient)
• iii) C N O F
• 4 u.p 3u.p 2u.p. 1u.p. up unpaired e-
• 4 bonds 3 bonds 2 bonds 1 bond
• OCO N??N O O F - F
• iv) H F are terminal in the structural formula
  (Never central)

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Atomic Connectivity (Bond Capabilities)

• The atomic arrangement for a molecule is usually
  given.
• CH2ClF HNO3 CH3COOH H2Se H2SO4 O3

In general when there is a single central atom in
the molecule, CH2ClF, SeCl2, O3 (CO2, NH3,
PO43-), the central atom is the first atom in the
chemical formula. Except when the first atom in
the chemical formula is Hydrogen (H) or fluorine
(F). In which case the central atom is the
second atom in the chemical formula. Find the
central atom for the following 1) H2O a) H b)
O 2) PCl3 a) P b) Cl 3) SO3 a) S b) O 4) CO32- a)
C b) O 5) BeH2 a) Be b) H 6) IO3- a) I b) O
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Lewis Dot Structure of CO2 by Bonds Table
A. Calculate Octet electrons and Total Valence
electrons to determine number of bonds
B. Calculate the number of bonds in compound
structure. (24- 16) 8 4
bonds 2 2 C. Calculate the remaining
electrons to add to structure to complete Lewis
dot structure. 16 - 8 8 e-Remaining
CO2 of e in its octet of valence e-
1 C 1(8) 8 1(4) 4
2 O 2(8)16 2(6)12
24 16
Total of 16e- in CO2, of which 8 electrons are
used to form 4 bonds and 8 remaining electrons
are used to complete Lewis structure.
1 Write atom connectivity for CO2.
2. Draw the four bonds in the structure.
3. Place the remaining 8 electrons in the
structure to complete the Lewis Structure
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Lewis Dot Structure of ClO4- by Bonds Table
A. Calculate
ClO4- of e in its octet of valence e-
1 Cl 1(8) 8 1(7) 7
4 O 4(8)32 4(6)24
Charge 1
40 32
B. Number of Bonds. bonds (40- 32) 8
4 bonds 2 2 C. Remaining
electrons. 32 - 8 24 e-Remaining
Writing Lewis Structure
3. Place the remaining 24 electrons in the
structure such that each atom has an octet to
complete the Lewis Structure
2. Draw the four bonds in the structure.
1. Write atom connectivity for ClO4-.
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Lewis Structures Examples

• Example
• a) CH2ClF b) SO2
• c) SO42- d) H3PO4

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Boot elements and the Lewis Dot Structures