LEWIS DOT DIAGRAMS AND MOLECULAR GEOMETRY
AND ANMOL P4 CHEM
Lewis Dot Diagrams
MANAV
Lewis Dot Diagrams are used to represent the covalent bonding of a molecule or ion.
The atoms shown in a Lewis Dot Diagram display the electrons found in the outer most shell of that specific atom.
The atoms in a Lewis dot structure tend to share electrons to complete each of the atoms' outer shell, by making these atoms have a total of 8 outer electrons, also known as valence electrons (with the exception of hydrogen, which will have a complete shell of 2 electrons). This is known as the octet rule .
How to build
When drawing Lewis structures, the valence electrons must be separated by first placing lone electrons (either on top, bottom, left, or right) around the element, then pairing any excess electrons. There should only be a maximum of 2 electrons on one side of the Lewis structure.
Example
When chemicals bond together, they share valence electrons. in Lewis structures, the sharing of electrons is represented by a straight line, connecting the electrons being shared.
In some cases, atoms can share more than one electron with one other electron. This is called a double bond. Triple bonds are also found, but are rarer.

Example of double bond:
Sometimes there are ions found in chemical equations. To represent these equations in lewis structures, just add or subtract electrons dependent on the ion.
Example : NH
4
+
Resonance Structures
Sometimes, there are several possible lewis structures for one chemical equation.
These different possibilities are called resonance structures. Resonance structures are usually found when there is a double or triple bond present. When drawing a lewis structure, and there are different possibilities for the structure, all resonance structures must be shown.
Example: NO
3
-

Formal Charge
To ensure that a lewis structure is proper, you can calculate the formal charge of each atom found in the structure. To calculate the formal charge of an atom, use the following equation :
Formal Charge = [ Number of Valence Electrons ] – [ Electrons in lone pairs + ½( number of bonding electrons) ]
If the formal charge of every atom in a lewis structure is 0, then the lewis structure is proper.
Practice Problems

c) CCl
4 b) BCl
3
1. Give the lewis structure for each of the following compounds: a) BeF
2

d) PBr
5 e) SI
6
2. Draw the lewis structures for the following ions: a) BH
2
–

b) NI
3 c) ClF
4
+ d) SF
5
–

Answers :
1.
a) b)

c) d)
2.
a) b) c)

d)
MODEL SPECIES
TYPE
AX
2
AX
3
SURROUNDING
ATOMS
2
LONE
PAIRS
0
3 0
SHAPE
Linear
Trigonal
Planar
BOND
ANGLE
180
120
AX
2
E
1
2 1 Bent <120
AX
4
4 0 Tetrahedral 109.5
AX
3
E
1
3 1 Trigonal
Pyramidal
107

AX
4
E
1
AX
3
E
2
AX
2
E
3
AX
6
AX
2
E
2
AX
5
2
5
2 Bent 104.5
0 Trigonal
Biprimidal
120 and 90
4
3
1 See-Saw <90 and
<120
2 T-shaped 90 and 180
2
6
3 Linear 180
0 Octahedral 90

AX
5
E
1
AX
4
E
2
AX
3
E
3
AX
2
E
4
5 1 Square
Pyramidal
<90
4
3
2
3
Square
Planar
T-shaped
90
90 and 180
3 4 Linear 180
HYBRID ORBITALS
NUMBER
OF
ORBITAL
S
NEEDED
2
HYBRIDIZATIO
N
Sp 2p
LEFT
OVER
ORBITAL
S
GEOMETRY EXAMPLE
S
BeF
2
,
HgCl
1

3
4
Sp 2
Sp 3
1p
0
BF
3
, SO
3
CH
4
, NH
3
,
H
2
O
PRACTICE PROBLEMS:
1.) Predict the geometries of the following using the VSEPR method
(a) PCL
3
: ________
(b) CHCl
3
: ________
(c) SiH
4
: ________
(d) TeCl
4
: ________
(e) HgBr
2
: ________
(f) N
2
O (arrangement of atoms is NNO): ________
(g) SCN- : ________
2.) If two compounds have the same molecular formulas, one with a linear structure, and the other with a branch, which one would you expect to have the higher boiling point and why?
3.) Predict the approximate bond angles in the following: a. GeCl 2 b. IF
4
- c. TeCl
4

ANSWER KEY
1.)Trigonal pyramid
Tetrahedral
Tetrahedron
Distorted Tetrahedron
Linear
Linear
Linear
2.)The compound with the linear structure because linear molecules get closer to one another because there are no branches separating them. Therefore, the intermolecular forces that is exerted is stronger the linear molecule, and a higher temperature is needed for it to boil.
3.)a. 120, b. 90, c. 90 and 120