Formal Charges

04/21/23Dr 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.

04/21/23Dr 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.

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.... ::

....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.

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.... ::

....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.

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.... ::

....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.

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.... ::

....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

+1.04/21/23Dr 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.

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.... ::

....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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04/21/23Dr Seemal Jelani Chem-241 13

Hydronium ion assigned 5 valence electrons

.. + Formal charge of +1

H O H

H 6-(2+3)

6-5 + 1

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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.

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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::.... ....