# Review of Lewis dot structure, molecular geometry and polarity

Document Sample

```					 Review of Lewis dot structure,
molecular geometry and polarity of
molecules

Ch pt 8: s ct 5, 6
Chapter    sect. 5
1-
Chapter 9: sect. 1-3
Electron Distribution in Molecules

Electron distribution is
depicted with Lewis electron
dot structures;;

Electrons are distributed as
shared or BOND PAIRS and
PAIRS.
unshared or LONE PAIRS.

G. N. Lewis
1875 - 1946
Bond and Lone Pairs
Electrons are distributed as shared or
BOND PAIRS and unshared or LONE
PAIRS.
PAIRS.
••
•
H     Cl    •
••
lone pair (LP)
shared or
bond pair

This is called a LEWIS ELECTRON
STRUCTURE.
DOT STRUCTURE
Building a Lewis Structure
Ammonia, NH3
1.   Decide on the central atom:
1.    Central atom tends to be the atom with the
Electronegativity.
lowest Electronegativity.
2.               H.
It’s never H.
Therefore,
Therefore N is central

2.   Count the total number of valence electrons
H=1 and N=5
(     )                        p
Total = (3 x 1) + 5 = 8 electrons or 4 pairs
Building a Lewis Structure

3.
3    Form a single bond between the central atom
and surrounding atoms.

H    N   H       Each bond line
represents two
electrons
H
Building a Lewis Structure
4.   Remaining electrons form LONE PAIRS to
needed.
complete octet as needed
but not if H;
place any leftover electrons on the
central atom

••

3 BOND PAIRS        H    N    H
and
1 LONE PAIR.             H
Lewis Structure: SO32-
Step 1. Central atom S
Step 2. Count valence electrons
S= 6
3 x O = 3 x 6 = 18
+2
Charge =
TOTAL = 26
single
Step 3. Form s ngle bonds
O
10 pairs of electrons
are now left.
O   S   O
Lewis Structure: SO32-
Step 4. Remaining pairs become lone pairs,
first on outside atoms and then on
central atom
••
•
•   O    •
•

••                ••
•
•   O
••
S
••
O
••
•
•

Each atom is surrounded by an
octet of electrons
Practice
Draw the Lewis dot structure of the
following
f ll i compounds:d

HBr
NI3
SiH4
H2O
Carbon Dioxide, CO2

1. Central atom =

2. Valence electrons = __ or __ pairs

3 Form single bonds.
3.            bonds

O      C   O
Carbon Dioxide, CO2
4. Place lone pairs on outer atoms.
••                ••
•                              •
•   O   ••
C        O   ••
•

5. So that C has an octet, we have to
form DOUBLE BONDS between C
and O.
••       ••
•                          •
•
•   O
••
C   O
••
•
•            •   O
••
C   O
••
•
Multiple Bonds
Double and even triple bonds are commonly
C N P O
observed for C, N, P, O, and S
H2CO

C2F4

SO3
Practice
Draw the Lewis dot structure of the
following
f ll i compounds:d

C2H4
F2CO
N2
CH3CN
CN-1
NOCl
Exceptions to the Octet Rule
There are three types of ions or molecules
that do not follow the octet rule:

Ions or molecules with an odd number of
electrons.

Ions or molecules with less than an octet.

Ions or molecules with more than eight valence
electrons (an expanded octet).
Exceptions to Octet Rule
Usually occurs with B, elements of higher periods,
d f bl
or compounds of noble gases.

Rules:
R l
If filling the octet of the central atom results in a
negative charge on the central atom and a positive
g           g                    m       p
charge on the more electronegative outer atom, don’t
fill the octet of the central atom.

When the central atom is on the 3rd row or below and
expanding its octet eliminates some formal charges, do
so.
Where are we going?

p
We learned how to represent electrons
distribution in a molecule using the
Lewis structure.

Today, we are going to discover how this
information provide a great deal of
molecules.
What do we need to answer
these questions?
Why do oranges smell like
oranges and not like lemons?

How d
H    do
proteins
do their job?
How does an
antibiotic
k ?
works?
Why is CO a
lethal gas?
Molecular compounds
The chemical properties of
compounds depend on the
nature of the atoms and
their arrangement in space
g          p
within a molecule.
VSEPR theory

p
Valence Shell Electron Pair Repulsion
Theory

The most important factor in determining
geometry is the relative repulsion
b t      l t        i s      d the     t l
between electron pairs around th central
atom.

minimizes the electron pair repulsions
VSEPR Rules: Electron Geometry
The electron geometry is identified
using the following rules:

.
1. Build Lewis structure of the molecule;

2. Count the number of electron pairs around
(nonbonding pair, single
the central atom (nonbondin pair sin le
bond and multiple bond all represent one
electron pair)

3. Identify the position in which the repulsion
minimum.
of bond pairs would be minimum
Practice

Identify the electron geometry of the
following molecules:

BeF2   Linear

BF3      Triangle: trigonal planar
CH4      Pyramide: Tetrahedral
Electron
Geometry
Geometry

Molecular geometry
Electron geometry:

Molecular geometry: arrangements of the
t      l
atoms only
Molecular geometry
Within each electron domain, then, there
might be more than one molecular
geometry.

g
Angle =120°
Molecular Geometry

Angle =109.5°
Molecular Geometry
Molecular Geometry
Variation in bond angles
Nonbonding pairs and multiple
pa rs     phys cally
bonds electron pairs are physically
larger than bonding pairs.
Therefore,
Therefore their repulsions are
greater; this tends to decrease
bond angles in a molecule.
Molecular Polarity
Molecular polarity is at the origin of the
molecules.
interaction between molecules

Understanding molecular polarity will h l
U d    t di        l   l     l it    ill help
us predict boiling point, melting point, etc.
Molecular Polarity
What is a dipole moment?
A dipole represents the difference in
electronegativity between two atoms

When did we talk about it?
When we talked about covalent bonding, we
introduced th concept of polar and non
i t d    d the      t f l        d
polar bonds
Is the molecule below going to be polar?
YES
A dipole moment is
represented by a
vector
Molecule Polarity
Electronegativity measure the affinity of
an atom to attract electrons to itself;
When there is a
difference in
electronegativity
between atoms ,
the bond is polar
and one atom pull
the electrons
toward itself more
th th r
than the other
Molecular polarity
If the different types of pulls cancel each
other the molecules is non polar;

don’t                 polar
But if they don t the molecule is polar.

Different factors come in:
Bonds polarity

Geometry of the molecule
Quiz
1. Identify the
1. Electronic geometry
1 El        i
2. Molecular geometry
3 B d angles
3. Bond      l
4. Polarity
th following polyatomic ion:
Of the f ll i p l t mi i :

SeO32-
Quiz

1.
1 Identify the
1.   Electronic geometry - tetrahedral
2.
2    Molecular geometry – trigonal pyramidal
3.   Bond angles - <109.5°
4.   Polarity - polar

O=Se-
O=Se-O

O

```
DOCUMENT INFO
Shared By:
Categories:
Stats:
 views: 835 posted: 7/30/2010 language: English pages: 35
How are you planning on using Docstoc?