LEWIS DOT DIAGRAMS AND MOLECULAR GEOMETRY
AND ANMOL P4 CHEM
MANAV
Lewis Dot Diagrams
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 : NH4+
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: NO3-
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
1. Give the lewis structure for each of the following compounds:
a) BeF2
b) BCl3
c) CCl4
d) PBr5
e) SI6
2. Draw the lewis structures for the following ions:
a) BH2–
b) NI3
c) ClF4+
d) SF5–
Answers :
1.
a)
b)
c)
d)
2.
a)
b)
c)
d)
Molecular Geometry
SPECIES
TYPE
AX2
MODEL
SURROUNDING LONE
ATOMS
PAIRS
2
0
SHAPE
AX3
3
0
Trigonal
Planar
120
AX2E1
2
1
Bent
<120
AX4
4
0
Tetrahedral
109.5
AX3E1
3
1
Trigonal
Pyramidal
107
Linear
BOND
ANGLE
180
AX2E2
2
2
Bent
104.5
AX5
5
0
Trigonal
Biprimidal
120 and 90
AX4E1
4
1
See-Saw
<90 and
<120
AX3E2
3
2
T-shaped
90 and 180
AX2E3
2
3
Linear
180
AX6
6
0
Octahedral
90
AX5E1
5
1
Square
Pyramidal
<90
AX4E2
4
2
Square
Planar
90
AX3E3
3
3
T-shaped
90 and 180
AX2E4
3
4
Linear
180
HYBRID ORBITALS
NUMBER HYBRIDIZATIO
OF
N
ORBITAL
S
NEEDED
2
Sp
LEFT
OVER
ORBITAL
S
2p
GEOMETRY
EXAMPLE
S
BeF2,
HgCl1
3
Sp2
1p
BF3, SO3
4
Sp3
0
CH4, NH3,
H2O
PRACTICE PROBLEMS:
1.) Predict the geometries of the following using the VSEPR method
(a) PCL3: ________
(b) CHCl3: ________
(c) SiH4: ________
(d) TeCl4: ________
(e) HgBr2: ________
(f) N2O (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. IF4- c. TeCl4
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