Download Molecular Structure:

SCC-CH110/UCD-CH41C
Chapter: 12
Instructor: J.T.
P1
Molecular Structure:
3-Dim arrangement of atoms in a molecule.
In Lewis Symbols:
The number of dots = number of valance electrons =
column number (main group/Periodic table)
-
Writing Lewis Structure: (i.e.: PO2F 2)
1) Draw a skeleton structure of molecule
(The less electronegative element is the central atom)
(H is always a terminal atom)
Example:
F
F
P
-
O
O
2) Determine the total number of valence electrons in the
structure.
Element/Charge
P
O
O
F
F
0ne negative charge
Total
Valence ē
5
6
6
7
7
1
32
SCC-CH110/UCD-CH41C
Chapter: 12
Instructor: J.T.
P2
3) Deduct the 2 valance electrons for each bound in step 1.
Distribute the remaining electrons as unshared pairs.
Each atom has 8 electroins if
possible.
If there are too few electroins
to give octet rule, convert
single bonds to multiple bonds.
F
-
O
P
F
O
12 lone pairs
4 electron pairs as 4 single bonds
(
)
Jupiter’s atmosphere contains small amount of compounds
methane, CH4, acetylene, C2H2 , and phosphine, PH3. What
are the Lewis structures of these molecules?
P
C
H
H
H
H
C
H
H
H
C
C
H
P
H
H
SCC-CH110/UCD-CH41C
Chapter: 12
Instructor: J.T.
P3
The three-dimensional arrangement of atoms in a
molecule is called molecular structure.
A bond distance:
Distance between the nuclei of two bonded atoms along a
straight line joining the nuclei (unit: picometer: 10-12 m).
A bond angle:
The angle between any two bonds that include a common
atom.
To predict approximate bond angles around a central atom in a
molecule by using VSEPR theory:
Valance Shell Electron-Pair Repulsion
The electrostatic repulsion of bond pair and unshared pairs
electrons is reduced to a minimum when they occupying
region of space as far from each other as possible.
180°
High electrostatic repulsion
Low electrostatic repulsion
120°
SCC-CH110/UCD-CH41C
Chapter: 12
Instructor: J.T.
P4
Polarity of bonds & polarity of molecules:
Polar bond:
A bond in which bonding electrons are shared unequally.
Low
High
Electronegativity
Electronegativity
A polar molecule has unbalanced polar covalent bonds.
O
C
O
δO
H
δ+
H
SCC-CH110/UCD-CH41C
Chapter: 12
Instructor: J.T.
P5
Organic Chemistry = Chemistry of Carbon compounds
Hydrocarbons: compounds made up of C & H
Name
# Of
C
Methane 1
Ethane
2
Propane 3
Butane 4
Alkane n
Formula
CH4
C 2H 6
C 3H 8
C4H10
Isomers: same formula but
different arrangements of
atoms.
(n-butane, isobutene)
C
C
C C
C
C C
C
CnH(2n+2)
There are 300,000
isomeric structures for
C20H42 !!
C is able to form four covalent bonds with other atoms.
Alcohols:
C
O
H
i.e.: methanol, ethanol, and propanol
SCC-CH110/UCD-CH41C
Ethers:
Chapter: 12
C
O
Instructor: J.T.
C
i.e.: dimethyl ether
O
Carboxylic acids
C
i.e.: formic acid, acetic acid
O H
P6