Formal Charges
04/19/23Dr Seemal Jelani 1
Formal charge is the charge calculated for an atom in a Lewis structure on the basis of an equal sharing of bonded electron pairs.
04/19/23Dr Seemal Jelani 2
Nitric acidNitric acid
We will calculate the formal charge for each atom in this Lewis structure.
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.... ::
....HH OO
OO
OO
NN
::
::....
....
Formal charge of HFormal charge of H
Nitric acidNitric acid
Hydrogen shares 2 electrons with oxygen.Assign 1 electron to H and 1 to O.A neutral hydrogen atom has 1 electron.Therefore, the formal charge of H in nitric
acid is 0.
04/19/23Dr Seemal Jelani 4
.... ::
....HH OO
OO
OO
NN
::
::....
....
Formal charge of HFormal charge of H
Nitric acidNitric acid
Oxygen has 4 electrons in covalent bonds.Assign 2 of these 4 electrons to O.Oxygen has 2 unshared pairs. Assign all 4
of these electrons to O.Therefore, the total number of electrons
assigned to O is 2 + 4 = 6.
04/19/23Dr Seemal Jelani 5
.... ::
....HH OO
OO
OO
NN
::
::....
....
Formal charge of OFormal charge of O
Nitric acidNitric acid
Electron count of O is 6. A neutral oxygen has 6 electrons. Therefore, the formal charge of O is
0.
04/19/23Dr Seemal Jelani 6
.... ::
....HH OO
OO
OO
NN
::
::....
....
Formal charge of OFormal charge of O
Nitric acidNitric acid
Electron count of O is 6 (4 electrons from unshared pairs + half of 4 bonded electrons).
A neutral oxygen has 6 electrons. Therefore, the formal charge of O is 0.
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.... ::
....HH OO
OO
OO
NN
::
::....
....
Formal charge of OFormal charge of O
Nitric acidNitric acid
Electron count of O is 7 (6 electrons from unshared pairs + half of 2 bonded electrons).
A neutral oxygen has 6 electrons. Therefore, the formal charge of O is -1.
04/19/23Dr Seemal Jelani 8
.... ::
....HH OO
OO
OO
NN
::
::....
....
Formal charge of OFormal charge of O
Nitric acidNitric acid
Electron count of N is 4 (half of 8 electrons in covalent bonds).
A neutral nitrogen has 5 electrons. Therefore, the formal charge of N is
+1.04/19/23Dr Seemal Jelani 9
.... ::
....HH OO
OO
OO
NN
::
::....
....
Formal charge of NFormal charge of N
––
Nitric acidNitric acid
A Lewis structure is not complete unless formal charges (if any) are shown.
04/19/23Dr Seemal Jelani 10
.... ::
....HH OO
OO
OO
NN
::
::....
....
Formal chargesFormal charges
––
++
Formal ChargeFormal ChargeFormal ChargeFormal Charge
Formal charge = Formal charge =
group numbergroup numberin periodic tablein periodic table
number ofnumber ofbondsbonds
number ofnumber ofunshared electronsunshared electrons
–– ––
An arithmetic formula for calculating formal charge.An arithmetic formula for calculating formal charge.
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"Electron counts""Electron counts" and formal and formal charges in NHcharges in NH44
+ + and BFand BF44--
11
44
NN
HH
HH HH
HH
++77
44
....
BBFF
FF
FF
FF
....
............:: ::
:: ::
:: ::
....
––
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04/19/23Dr Seemal Jelani Chem-241 13
Hydronium ion assigned 5 valence electrons
.. + Formal charge of +1
H O H
H 6-(3+2)
6-5 + 1
04/19/23Dr Seemal Jelani Chem-241 14
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Condensed structural formulas
Lewis structures in which many (or all) covalent bonds and electron pairs are omitted.
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HH
OO
CC CC CC
HH HH HH
HH
HHHH :: ::
HH
can be condensed to:can be condensed to:
CHCH33CHCHCHCH33
OHOH
(CH(CH33))22CHOHCHOHoror
Bond-line formulas
Omit atom symbols. Represent structure by showing bonds between carbons and atoms other than hydrogen.
Atoms other than carbon and hydrogen are called heteroatoms.
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CHCH33CHCH22CHCH22CHCH3 3 is shown asis shown as
CHCH33CHCH22CHCH22CHCH22OHOH is shown asis shown as
OHOH
Bond-line formulas
Omit atom symbols. Represent structure by showing bonds between carbons and atoms other than hydrogen.
Atoms other than carbon and hydrogen are called heteroatoms.
04/19/23Dr Seemal Jelani 18
HH ClClCC
CC
HH22CC
HH22CC
CHCH22
CHCH22
HHHH
is shown asis shown as
ClCl
Constitutional Isomers
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Constitutional isomers
Isomers are different compounds that have the same molecular formula.
Constitutional isomers are isomers that differ in the order in which the atoms are connected.
An older term for constitutional isomers is “structural isomers.”
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A Historical Note
In 1823 Friedrich Wöhler discovered that when ammonium cyanate was dissolved in hot water, it was converted to urea.
Ammonium cyanate and urea are constitutional isomers of CH4N2O.
Ammonium cyanate is “inorganic.” Urea is “organic.” Wöhler is credited with an important early contribution that helped overturn the theory of “vitalism.”
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NHNH44OCNOCNAmmonium cyanateAmmonium cyanate
HH22NCNHNCNH22
OO
UreaUrea
NitromethaneNitromethane Methyl nitriteMethyl nitrite
.... ::
HH CC
OO
OO
NN
::
::....
––
++
HH
HH
Examples of constitutional isomers
Both have the molecular formula CH3NO2 but the atoms are connected in a different order.
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....CC OO NN OOHH
HH
HH
....::.... ....
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Shapes
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Methane
tetrahedral geometryH—C—H angle = 109.5°
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Methane
tetrahedral geometryeach H—C—H angle =
109.5°
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Valence Shell Electron Pair Repulsions
The most stable arrangement of groups attached to a central atom is the one that has the maximum separation of electron pairs(bonded or nonbonded).
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Water
bent geometryH—O—H angle = 105°
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but notice the tetrahedral arrangement but notice the tetrahedral arrangement of electron pairsof electron pairs
OO
HH
....
HH
::
Ammonia
trigonal pyramidal geometryH—N—H angle = 107°
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but notice the tetrahedral arrangement but notice the tetrahedral arrangement of electron pairsof electron pairs
NNHH
HH
HH
::
Boron Trifluoride
F—B—F angle = 120°trigonal planar geometry
allows for maximum separation
of three electron pairs
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Formaldehyde: CH2=O
H—C—H and H—C—Oangles are close to 120°
trigonal planar geometry
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CC OO
HH
HH
Figure 1.12: Carbon Dioxide
O—C—O angle = 180°linear geometry
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OO CC OO
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Polar Covalent Bonds and Electronegativity
Electronegativity is a measure of an element to attract electrons toward itself when bonded to another element. An electronegative element attracts electrons. An electropositive element releases electrons.
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1.0
Na
0.9
Li Be B C N O F
1.5
Mg
1.2
2.0
Al
1.5
2.5
Si
1.8
3.0
P
2.1
3.5
S
2.5
4.0
Cl
3.0
Pauling Electronegativity ScalePauling Electronegativity Scale
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Electronegativity increases from left to right in the periodic tableElectronegativity decreases going down a group.
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Generalization
Nonpolar bonds connectNonpolar bonds connectatoms of the same atoms of the same electronegativityelectronegativity
H—HH—H ::NN NN::FF::........FF::
....
....
Generalization
The greater the difference in electronegativitybetween two bonded atoms; the more polar the
bond.
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polar bonds connect atoms ofpolar bonds connect atoms ofdifferent electronegativitydifferent electronegativity
::OO CC
FF::........HH
OO........HH
HH
OO::.... ....