How to write lewis structure

Writing Lewis StructuresWriting Lewis Structures

Rules for writing structures

(1) you must know the order in which theatoms are connected

This is normally determined by experimentand is referred to as the constitution of amolecule

General rule: the atom with the lowest electronaffinity is usually the central atom

Elemental Composition

Levels of Structure

Empirical Formula

Molecular FormulaConstitution

ConfigurationConformation


(1) you must know the order in whichthe atoms are connected

This is normally determined by experiment and isreferred to as the constitution of a molecule

Example: hypochlorous acid has themolecular formula HClO. But the atoms areconnected in the order of HOCl


(2) Count the number of valence electrons

For main group elements this is the same as thegroup number in the periodic table

Example: hypochlorous acid : HOCl

H 1 electron O 6 electron Cl 7 electron

total 14 valence electrons


(3) write out the constitution in a formthat shows the covalent bonds and countthe number of electrons in covalent bonds

4 electrons in covalent bonds

14 valence electrons

10 electrons remain to be assigned

Example: hypochlorous acid H O Cl


(4) assign remaining electrons so as tocomplete the octets of as many atoms aspossible.

Example: hypochlorous acid HOCl

(4 electrons in covalent bonds + 10 moreelectrons assigned as shown)

H O Cl ::

:

::


(5) when the number of electrons isinsufficient to complete the octetes of all ofthe atoms, assign them to atoms in order ofdecreasing atom electronegativity.

Example: nitrous acid HNO2 ( HONO )

Need to assign 12 electons in addition to 6 foundin three bonds


H O N O

::

::

: :


(6) use unshared pairs for double bonds ifthis will satisfy octet rule.

Example: nitrous acid HONO

O:

H O N :

:: : :

H O N :

:: : O

:

Another example


Example: hyponitrous acid HONNOH

H O N N O H

: ::: :

H O N N O H

:

:

Another example


H O N N O H

: ::: :

H O N N O H

:

Example: hyponitrous acid HONNOH

Ions

Ammonium ion ( NH4+ )

Subtract one electron for each positivecharge

Number of electrons = 5 + 4 -1 = 8

N H

H

H

H+

Ions

Tetrafluoroborate ( BF4- )

add one electron for each negative charge

Number of electrons = 3 + 28+1 = 32

B F

F

F

F-

:

::

::

:::

:

:::


(7) assign formal charges

+

-

Example : nitric acid HNO3 (HONO2)

:

O

:

:

N

O

O

::

:

:

H

Formal Charge and LewisStructure

Formal Charge and LewisStructure

Formal ChargeFormal Charge

=number of valenceelectrons in neutralatom

- electron countof atom

electron count =number of

electrons“owned” by atom

+ one- half the

number of sharedelectrons

electron counts and formal charges inNH4

+ and BF4-

-B F

F

F

F

:

:

:

:

:

:

::

:::

:

N H

H

H

H

+

4 4

1 7

“Electron counts” in nitric acid

:

O

:

:

N

O

O

::

:

:

H

1

6 7

4

6

Formal charges in nitric acid

:

O

:

:

N

O

O

::

:

:

H

7

4

1+

1-

Lewis Structure of nitric acid

:

O

:

:

N

O

O

::

:

:

H +

-

Formal Charge

does not represent the real charge on anatom in the molecule

it can however be used to determine thevalidity of a molecules Lewis structure

Try to minimize formal charge in your Lewisstructures

Avoid positive values of formal charge onhighly electronegative elements

Recall :Recall :

Rules for writing Lewis Structures

Example : nitrous acid HONO

(6) use unshared pairs for double bonds if thiswill satisfy octet rule.

:

H O N :

:: : :

O H O N :

:: : O

:

:H O N :

::

:

:

O

:

H O N ::

:

O

:

+ -

H O N :

:: : O

:

:

H O N :

:

:

O

:

--++

More stable LewisStructure

Less stable LewisStructure

Formal Charge

Formal Charge

The sum of the formal charges of allatoms in a given molecule or ion mustequal the overall charge on that species

Two Conventions

formal charges are closer to actualatomic charges than are oxidation states

Oxidation States

Formal Charges

but are still only estimations ofmolecular atomic charge

Lewis Structure of nitric acid

:

O

:

:

N

O

O

::

:

:

H

formal charge

+

-

oxidation state

+5

-2oxidation state = electrons lost or gainedplus formal charge

The Concept of ResonanceThe Concept of Resonance

Resonance

Two or more Lewis structures may bewritten for certain compounds (or ions )

Recall :Recall :


Example :bicarbonate HOOCO -


-

: :

: :::

C

O

OO

:

H

:

Recall :Recall :


Example :bicarbonate HOOCO -


-

: :

: :::

C

O

OO

:

H

C

O

OO H

: :

:

::

-

Electrons in molecules are often delocalizedbetween two or more atoms.

What writing resonance structuresaccomplishes

electrons in a single Lewis structure are assignedto specific atoms

a composite of resonance forms more accuratelydepicts electron distribution

a single Lewis Structure is insufficient to showelectron delocalization.

“localization”.

Example

: :

: :::: +

-

N

O

OO

:

-:

+ N

O

OO

::

:

:

:::

-

-

:

O

:

N

OO

::

: ::

:

+

-

-

Nitrate ionNitrate ion

Exceptions to the octet ruleExceptions to the octet rule

Counting only valence electrons

elements in the second period can neverhave more than 8 electrons

but can have fewer than 8

elements in the third period can havemore than 8 electrons

Less than 8 electrons

F

F: ::

B

F

::

::

:

:

more than 8 electrons

S

F

:

:

:

F:::

F

:

:

:

F

:

:

:

F:

:: F ::

:

Coordinate Covalent Bond

+ N H

H

H

:B

:

: F

:: F

:

:

:

: F :

A covalent bond in which one of the atomsdonates both electrons

Coordinate Covalent Bond

N H

H

H

B

:

: F

:: F

:

:

:

: F :

+-

A covalent bond in which one of the atomsdonates both electrons

The distinction is useful for keeping track ofelectrons and assigning formal charge

Odd Electron Molecules

Some molecules contain an odd number ofelectrons

The octet is not complete

( NO )

N: .O

: :

and ( NO2 ) notable examples

-N: .O

:

:

:O

:+

Date post:	27-May-2015
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How to write lewis structure

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