lewis structures ©2011 university of illinois board of trustees

Lewis Structures

©2011 University of Illinois Board of Trustees • http://islcs.ncsa.illinois.edu/copyright

Lewis Structures

•Are models

•The representations of the electron arrangements in atoms, ions, or molecules by showing the valence electrons as dots placed around the symbols for the elements

•Also called Lewis Dot Diagrams or Electron Dot Diagrams

•Can be drawn for atoms, molecules, anions, cations or ionic compounds

•Useful when determining the geometry or shape of a molecule


Lewis Structures

Why are they important?

•You can visualize the electrons involved in chemical bonds

•You can gain a greater understanding of how chemical bonds form


Lewis Structures

1) Count up total number of valence electrons

2) Decide on arrangement of atoms and connect all atoms with single bonds

- “least electronegative atoms usually in the middle

- “single” atoms usually in center;

C always in center,

H always on outside.3) Complete octets on exterior atoms, lone pair electrons (not H, though)

4) Check:

- valence electrons math with Step 1

- all atoms (except H) have an octet; if not, try multiple bonds

- any extra electrons? Put on central atom


Lewis Structures

How Are They Drawn?

1) Count up all of the valence electrons for each atom in the formula for anions, add the charge of the ion to the number of valence electrons for cations, subtract the charge of the ion from the number of electrons2) Determine the number of octet electrons the formula should have

3) Determine the Number of Bonding Electrons Subtract valence electrons from octet electrons

4) Determine the Number of Bonds Divide # Bonding electrons by 2

5) Draw the Structure with the correct Number of Bonds, least electronegative element is usually the central atom Bond all atoms together by single bonds, then add in the multiple bonds until the rules in the notes are followed

6) Determine number of Lone Pair Electrons and arrange them around the atoms until the octet rule is satisfied for all atoms (except Hydrogen)


Some Examples

CO2

1) Valence Electrons = 16 Carbon is in group 4, 4 valence Oxygen is in group 6, 6 valence X 2 atoms = 12 total 4 + 12 = 16

2) Octet Electrons = 8 ea. X 3 = 24

3) Bonding Electrons = 24 – 16 = 8

4) Number of Bonds = (8/2) = 4

5) O = C = O

6) Non bonding electrons = 16 – 8 = 8©2011 University of Illinois Board of Trustees • http://islcs.ncsa.illinois.edu/copyright

Carbon Dioxide

This Lewis Structure uses the correct

number of electrons, but does not obey

the octet rule.

This Lewis Structure uses

the correct number of electrons

and obeys the octet rule.

O = C = O


Hydrogen Cyanide

HCN

1)Valence electrons = 10

2)Octet electrons = 18

3)Bonding electrons = 8

4)Number of Bonds = 4

5)H – C N

6)Number of Nonbonding electrons = 2

H – C N:


Exceptions to the Octet Rule (there are always exceptions)

BH3Each hydrogen accommodates 2 electrons, or one bond. The boron atom in BH3, on the other hand, has only 6

total electrons. Because boron is a smaller atom, it does not have

enough space to accommodate a full octet of 8 electrons.


BF3

BF3 has two potential Lewis structures shown below.Left structure (Structure I) is better because it minimizes

interaction between molecules.Structure II shows B and F with formals charges.F is a more electronegative atom (attracts more electrons)

and will have more 3 lone pairs, shown in Structure I.


Formal Charge

Determines which Lewis structure is correct if many structures are possible

Compares number of electrons around a bonded atom to the number of electrons a lone atom possessesFormal charge = #valence electrons – (#nonbonding + ½

bonding)The best Lewis Structure is the one where the

formal charges are as close as possible to zero


Figure I: Formal charge on top Fluorine is 0. Formal charge on right Fluorine is 0. Formal charge on left fluorine is 0. Formal charge on Boron is 0.

Figure II: Formal charge on top Fluorine is 0. Formal charge on right Fluorine is 1. Formal charge on left fluorine is 0. Formal charge on Boron is -1.


NO – Nitric Oxide

Exception: odd number of electronsConsider NO – 11 valence electronsBest course of action:

Maximize number of bond Make sure neither atom in the 2nd period exceeds an octet

One atom will have an odd electron count


1st 2nd or 3rd period elements as central atoms

Expanded octets: an exceptionAtoms in the 3rd period or higher can old more than 8

electronsThey can hold 8, 10, or 12 electrons around the central

atomExamples: XeF4, SF6, PCl5


Bonding and Shapes of Molecules

Number of Bonds

Number of Unshared Pairs on Central Atom

Shape Examples

2

3

4

3

2

0

0

0

1

2

Linear

Trigonal planar

Tetrahedral

Trigonal Pyramidal

Bent

BeCl2CO2

BF3

CH4, SiCl4, CCl4

NH3, PCl3

H2O,

-Be-=C=

B

C

N

:

O

:

:

CovalentStructure


Linear

BeCl2


Carbon dioxide

Lineargeometry

Linear


Trigonal Planar

BF3


Tetrahedral

Methane –The first member of the paraffin (alkane) hydrocarbons series. a.k.a. (marsh gas, CH4).

Methane


Tetrahedral

SiCl4 Silicon tetrachloride


Tetrahedral

Carbon tetrachloride – “carbon tet” had been used as dry cleaning solventbecause it is extremely non-polar.

Carbon tetrachloride – CCl4


Pyramidal

NH3

..

HH

H

N

Ammonia


Trigonal Pyramidal

Phosphorus trichloride


Bent

....

H HOWater

SO2

(-)

(+)

Polar molecule


lewis structures ©2011 university of illinois board of trustees

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