Transcript No Slide Title

Chemical bonding-Introduction
A.
Bond - refers to the attractive forces that hold atoms
together in compounds
B.
ionic bonding - transfer of electrons from one atom to
another
Remember - Ions are atoms or groups of atoms with a charge
1.
produces ionic compounds
a.
solids with high melting points
b
act as electrolytes when dissolved in
water or are melted
Electrolytes are substances that conduct electricity
when melted or dissolved
C.
covalent bonding - sharing of electrons
1.
produces covalent compounds
a.
gas, liquids or solids with low melting points
b.
generally poor electrolytes
electron shells
Atomic number = number of Electrons
Electrons vary in the amount of energy they possess,
and they occur at certain energy levels or
electron shells.
Electron shells determine how an atom behaves
when it encounters other atoms
Octet Rule = atoms tend to gain, lose or share electrons so
as to have 8 electrons
Carbon has 4 valence electrons
Needs 4 electrons
Needs 3 electrons
Nitrogen has 5 valence electrons
Needs 2 electrons
Oxygen has 6 valence electrons
Chemical Bonding
Compounds are formed from chemically bound atoms or ions.
Bonding involves only the valence electrons.
Valence electrons are the ones in the quantum shell with largest
n value
Use the periodic chart to guide determination of valence electrons
11-1 Lewis Theory: An Overview
• Valence e- play a
fundamental role in
chemical bonding.
• e- transfer leads to
ionic bonds.
• Sharing of e- leads to
covalent bonds.
• e- are transferred or
shared to give each
atom a noble gas
configuration
– the octet.
Lewis Symbols
• A chemical symbol represents the
nucleus and the core e-.
• Dots around the symbol represent
valence e- .
•N•
•
•
•
Al •
•
•
••
• Se
•
•
••
P•
••
• As •
•
•
••
I
••
•
• Sb •
•
••
Ar
••
Si •
••
••
••
••
•
•
••
••
• Bi •
•
Electron Dot Structures
Symbols of atoms with dots to represent the valence-shell
electrons
1
2
13
14
15
16
17
H
18
He:

Li  Be

B 

Na Mg



Al


C


N



O




 Si 
P
S 





: F  :Ne :




:Cl  :Ar :


II.
Ionic bonding
A.
formation of ionic compounds
1.
attraction of oppositely charged atoms
2.
anions are negative cations are positive
3.
opposites attract
4.
easy rule of thumb- the farther they are on the
periodic table - the more likely they will form
ionic bonds
B.
metals (group I) and nonmetals (group VII)
1.
the metals tend to loss electrons (oxidation)
2.
the nonmetals to gain electrons (reduction)
Na+1Cl-1
The Na atom become isoelectronic with Neon and Cl becomes
isoelectronic with argon
Isoelectronic - same electron configuration
III.
Covalent Bonding
A.
Formed when two atoms share two or more pairs of electrons
B.
product of a covalent bond is known as a molecule
C.
Covalent bonds themselves are fairly strong
D.
The attractions between the molecules is relatively weak
E.
small difference in electronegativity
1.
electronegativity - ability to attract a shared pair
of electrons
F.
Can share
1 electron - single, 2 electrons = double
3 electrons - triple
C
C
C
H
C
C
C
Octet rule
1.
In most cases, bonding elements achieve noble gas
configuration or become isoelectronic to a noble gas
2.
Finding number of Bonds
a.
S = N-A
The Octet Rule
Atoms tend to gain, lose, or share
electrons until they have eight
valence electrons.
Hydrogen is an
exception. It
shares only one
electron to reach
an outer shell of
two electrons
Ionic and Molecular Compounds
•Formation of sodium chloride: Ionic

 Na+ [ Cl

]


Cl

Na  +



Formation of Hydrogen chloride: (covalent)


H Cl



Cl

H +



A metal and a nonmetal transfer electrons to form
an ionic compound. Two nonmetals share
electrons to form a molecular compound.
Lewis Structures for Ionic
Compounds
Ba
• O•
••
O
••
2-
••
Ba•
2+ ••
••
••
BaO
•
••
••
Mg
••
• Cl
••
••
2 Cl
••
-
••
2+
••
Mg •
••
MgCl2
•
• Cl
••
Ionic Compounds
Ionic compounds consist of a lattice of
positive and negative ions.
NaCl:
Electronegativity Difference
5
If the difference in electronegativities is between:
–
–
–
1.7 to 4.0: Ionic
0.3 to 1.7: Polar Covalent
0.0 to 0.3: Non-Polar Covalent
The type of bond can usually be calculated by4
finding the difference in electronegativity of the
two atoms that are going together.
Example: NaCl
Na = 0.9, Cl = 3.0
Difference is 2.1, so
this is an ionic bond!
Bond Polarity
A polar bond can be pictured using
partial charges:
+

H
Cl
2.1
 = 0.9
3.0
Electronegativity
Difference
Bond Type
0 - 0.5
Nonpolar
0.5 - 2.0
Polar
2.0 
Ionic
Classify the following bonds as ionic, polar covalent,
or covalent: The bond in CsCl; the bond in H2S; and
the NN bond in H2NNH2.
Cs – 0.7
Cl – 3.0
3.0 – 0.7 = 2.3
Ionic
H – 2.1
S – 2.5
2.5 – 2.1 = 0.4
Polar Covalent
N – 3.0
N – 3.0
3.0 – 3.0 = 0
Covalent
9.5
Lewis Structures
For Covalent Bonds
H +  H  H H
H 2:
Represents
a pair of e-
or H H
Bonding
electrons






Nonbonding electrons


Cl
Cl
 





or
Cl
Cl
Cl
+
Cl





Cl2:

A covalent bond is a chemical bond in which two or more
electrons are shared by two atoms.
Why should two atoms share electrons?
F
+
7e-
F
F F
7e-
8e- 8e-
Lewis structure of F2
single covalent bond
lone pairs
F
F
lone pairs
single covalent bond
lone pairs
F F
lone pairs
9.4
Skeletal Structure
Hydrogen atoms are always terminal atoms
Central atoms are generally those with the lowest electronegativity.
Carbon atoms are always central atoms.
Generally structures are compact and symmetrical.
Skeletal Structure
• Identify central and terminal atoms.
H
H C
H
H
C O H
H
Lewis Structures
Draw Lewis structures for:
HF
H2O:
NH3:
CH4:


or

H O
H

or

H

H C
H

H

CH4:
H N H

H

NH3:


or
or


H F

H2O:


HF:

Lewis Structures
H F


H O
H


H N H
H
H
H C H
H
Lewis structure of water
H
+
O +
H
single covalent bonds
H O H
or
H
O
H
2e-8e-2eDouble bond – two atoms share two pairs of electrons
O C O
or
O
O
C
double bonds
- 8e8e- 8ebonds
double
Triple bond – two atoms share three pairs of electrons
N N
triple
bond
8e-8e
or
N
N
triple bond
9.4
Double and Triple Bonds
• Atoms can share four electrons to form a
double bond or six electrons to form a triple
bond.


N2:
N N

O
O
=



O 2:
• The number of electron pairs is the
bond order.
Multiple Covalent Bonds
•
•
••
N N
••
•
•
•
N N
••
••
••
•
•
N N
••
•N
•
N•
••
•
••
•
Multiple Covalent Bonds
•O
••
••
•
•
•
••
C O
O
••
••
•
•
•
•
O
C• O
••
••
••
• C•
•
••
••
O•
•
••
••
••
••
O C O
••
•
••
•
Strategy for
Writing Lewis
Structures
Drawing Lewis Structures
• .
• Draw a skeleton structure with atoms attached
by single bonds.
• Complete the octets of atoms bound to the
central atom.
• Place extra electrons on the central atom.
• If the central atom doesn’t have an octet, try
forming multiple bonds.
SAMPLE PROBLEM 1
Write a Lewis structure for CCl2F2, one of the compounds
responsible for the depletion of stratospheric ozone.
SOLUTION:
:
:
: Cl :
:Cl C F :
:F :
:
Make bonds and fill in remaining valence electrons
placing 8e- around each atom.
F
F
:
Steps 2-4:
C has 4 valence e-, Cl and F each have 7. The sum is 4
+ 4(7) = 32 valence e-.
Cl C
:
Step 1: Carbon has the lowest EN and is the central atom.
The other atoms are placed around it.
Cl
:
PROBLEM:
Writing Lewis Structures for Molecules with
One Central Atom
SAMPLE PROBLEM 2
Hydrogen can have only one bond so C and O must be next
to each other with H filling in the bonds.
There are 4(1) + 4 + 6 = 14 valence e-.
C has 4 bonds and O has 2. O has 2 pair of nonbonding e-.
H
:
SOLUTION:
Write the Lewis structure for methanol (molecular formula
CH4O), an important industrial alcohol that is being used as a
gasoline alternative in car engines.
H
C
O
:
PROBLEM:
Writing Lewis Structure for Molecules with
More than One Central Atom
H
H
Drawing Lewis Structures




Cl
C Cl



14 ve’s


HOCl
24 ve’s

COCl2


O
H O
Cl




CH 3OH
14 ve’s


26 ve’s
O
Cl
O




ClO 3



O
H

H C O
H

H
Third type of bonding Ionic, covalent and Metallic
Metalic bonding
Think of a metalic bond having a sea of electrons above it and below it
This sea of electrons gives metals their metallic properties
Conductivity
e- e- e- e- e- e- e- ee-
e-
e- e- e- e- e - e- e- e-
malleability
e-
e-
e- e- ee
e- e e e-
SAMPLE PROBLEM 3
PROBLEM:
PLAN:
Writing Lewis Structures for Molecules with
Multiple Bonds.
Write Lewis structures for the following:
(a) Ethylene (C 2H 4 ), the most important reactant in the
manufacture of polymers
(b) Nitrogen (N2 ), the most abundant atmospheric gas
For molecules with multiple bonds, there is a Step 5 which follows the
other steps in Lewis structure construction. If a central atom does not
have 8e-, an octet, then e- can be moved in to form a multiple bond.
SOLUTION:
C
:
H
:
H
(a) There are 2(4) + 4(1) = 12 valence e- . H can have only
one bond per atom.
C
H
H
H
C
H
H
C
H
(b) N2 has 2(5) = 10 valence e-. Therefore a triple bond is required to make
the octet around each N.
N
N
N
N
N
N
.
.
.
.
:
:
:
:
: .
.:
ball and stick model
SF4
Space filling Model