Molecular Formulas

1
Molecular Formulas

• consist of the symbol of the elements involved
  and the number of each
• e.g. H2O 1 molecule of water
• 2 atoms H 1 atom O
• 2H2O 2 molecules of water

subscript atoms in 1 molecule
x HyO
coefficient molecules
e.g. 2 Cl 2 atoms of chlorine
Cl2 1 molecule of chlorine
Cu(NO3)2 x of whatever is inside ( )x
i.e. 1 molecule has 1 atom of Cu 2 atoms of N
and 6 atoms of O
2
Valence Electrons
determine the chemical-character of an
element---these are the electrons gained, lost,
or shared in a chemical reaction. for the Main
Group elements, the valence electrons are the s
and p-electrons in the outermost shell (equals
the group ).
use dots to represent valence electrons
for the transition metals, the valence electrons
are those in the outermost ns and (n-1)d subshells
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Lewis Dot Structures
pairs of dots (lines) between atoms are
bonds. multiple pairs of dots represent multiple
bonds. valence electrons not involved in bonds
are usually in pairs.
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Covalent Bonding

• involves the sharing of valence electrons between
  atoms
• separation of nuclei is balance between
  electron-nuclear attraction, and
  electron-electron nuclear-nuclear repulsions
• OCTET RULE
• each atom in a molecule attempts to bond such
  that it achieves a Noble Gas configuration
• F F N N
• only 2 electrons per covalent bond can have more
  than one bond between nuclei

. . . . . . . .
5
Writing Lewis Dot Structures

• Write skeleton structure for molecule
• element with the least negative electron affinity
  at center
• use formula a hint
• Hs always, and Halides most often, terminal
  (NEVER form multiple bonds)
• Oxygens do not like to bond together (with a few
  exceptions)
• Determine number of valence electrons in molecule
• cations () subtract electrons
• anions (-) add electrons
• From this total, subtract 2 electrons for each
  bond in skeleton structure
• Distribute remaining valence electrons so that
  all atoms have complete octet
• may need to introduce multiple bonds

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Writing Lewis Dot Structures Example
SO3

• Write skeleton structure for molecule
• element with the least negative electron affinity
  at center
• use formula a hint
• Hs always, and Halides most often, terminal
  (NEVER form multiple bonds)
• Oxygens do not like to bond together (with a few
  exceptions)

O S O O
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Writing Lewis Dot Structures Example
SO3

• Determine number of valence electrons in molecule
• cations () subtract electrons
• anions (-) add electrons

Oxygen 6 x 3 18 e- Sulfur 6 x 1 6
e- Total 24 e-
O S O O
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Writing Lewis Dot Structures Example
SO3

• From this total, subtract 2 electrons for each
  bond in skeleton structure

Oxygen 6 x 3 18 e- Sulfur 6 x 1 6
e- Total 24 e- - Bonds 2
x 3 6 e- Electrons Left 18 e-
O S O O
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Writing Lewis Dot Structures Example
SO3

• Distribute remaining valence electrons so that
  all atoms have complete octet
• may need to introduce multiple bonds

Oxygen 6 x 3 18 e- Sulfur 6 x 1 6
e- Total 24 e- - Bonds 2
x 3 6 e- Electrons Left 18 e-
wants 6 more electrons
wants 2 more electrons
O S O O
wants 6 more electrons
wants 6 more electrons
There is a need of 20 electrons, but there are
only 18 left, so 2 must be shared---double bond.
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Writing Lewis Dot Structures Example
SO3

• Distribute remaining valence electrons so that
  all atoms have complete octet
• may need to introduce multiple bonds

• Oxygen 6 x 3 18 e-
• Sulfur 6 x 1 6 e-
• Total 24 e-
• - Bonds 2 x 3 6 e-
• Electrons Left 18 e-
• - Bond 2 x 1 2 e-
• Electrons Left 16 e-

wants 6 more electrons
wants 0 more electrons
O S O O
wants 4 more electrons
wants 6 more electrons
There is a need of 16 electrons, and there are
16 left, so distribute as lone pairs.
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Writing Lewis Dot Structures Example
SO3

• Distribute remaining valence electrons so that
  all atoms have complete octet
• may need to introduce multiple bonds

• Oxygen 6 x 3 18 e-
• Sulfur 6 x 1 6 e-
• Total 24 e-
• - Bonds 2 x 3 6 e-
• Electrons Left 18 e-
• - Bond 2 x 1 2 e-
• Electrons Left 16 e-

O S O O




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Exceptions to Octet Rule

• Hydrogen 2 electrons
• Group IIIA elements like 6 electrons e.g. BF3
• electron deficient or incomplete octets
• Elements of atomic number 14 and greater MAY have
  expanded octets (10 or 12 e-s)

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Formal Charges

• O
• O N OH
• simple means of bookkeeping
• the and - signs do not necessarily indicate
  actual charges
• negative charge on more electronegative atom
• sum of Formal Charges charge on molecule or ion
• Electroneutrality principle formal Charges are
  to be avoided whenever possible
• e.g. draw 2 structures for CHN, which actually
  exists?
• H C N H N C

F.C. valence electrons - nonbonding electrons
- bonds
.. ..
.. ..
N 5 - 0 - 4 1 O 6 - 6- 1 -1


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Bond Lengths

• distance between adjacent nuclei
• H H H H
• H C C H C C
  H C C H
• H H H H
• ethane ethylene
  acetylene
• Trends
• bond lengths shorten on going from single to
  double to triple bond (more electrons between
  nuclei)
• little change in single bond lengths (similar
  bond types have similar bond lengths)

0.133nm
0.110nm
0.120nm
0.154nm
0.109nm
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Resonance

• in many cases there are multiple equivalent
  places to put multiple bonds
• O
  O
• e.g. HNO3 O N OH
  O N OH
• note all N-O bond lengths are identical but
  neither single nor double.
• if 2 or more Lewis structures exist using the
  same backbone, then resonance is said to exist
• the individual structures contributing
  structures
• the molecule itself is a resonance hybrid of the
  contributing structures
• represent hybrid by drawing contributing
  structures and separating them by a
• the resonance hybrid is a blend (average) of the
  contributing structures
  O
• 
  O N OH

-
-


-
bond lengths equivalent to a 1 1/2 bond

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