When Atoms Meet

1
When Atoms Meet
2
Bonds

• Forces that hold groups of atoms
• together and make them function
• as a unit.

Bonding Forces

• Electron electron
• repulsive forces

• Nucleus nucleus
• repulsive forces

• Electron necleus
• attractive forces

3
Metals and Nonmetals
4
Types of Chemical Bonding
1. Metal with nonmetal
electron transfer and ionic bonding
5
Three models of chemical bonding
Ionic
Electron transfer
6
Types of Chemical Bonding
1. Metal with nonmetal
electron transfer and ionic bonding
2. Nonmetal with nonmetal
electron sharing and covalent bonding
7
Three models of chemical bonding
Ionic
Covalent
Electron transfer
Electron sharing
8
Types of Chemical Bonding
1. Metal with nonmetal
electron transfer and ionic bonding
2. Nonmetal with nonmetal
electron sharing and covalent bonding
3. Metal with metal
electron pooling and metallic bonding
9
Three models of chemical bonding
Ionic
Covalent
Metallic
Electron transfer
Electron sharing
Electron pooling
10
Valence Electrons

• The outer shell electrons of an atom
• Participate in chemical bonding

9.1
11
Lewis Structures
Developed the idea in 1902.
G. N. Lewis
12
Lewis Dot Symbols
Nitrogen, N, is in Group 5A and therefore has 5
valence electrons.
13
Lewis Dot Symbols
14
The Octet Rule
Chemical compounds tend to form so that each
atom, by gaining, losing, or sharing electrons,
has eight electrons in its highest occupied
energy level.
The same number of electrons as in the nearest
noble gas
The first exception to this is hydrogen, which
follows the duet rule.
The second exception is helium which does not
form bonds because it is already full with its
two electrons
15
Ionic Bond
He
Ne
1s22s1
1s22s22p5
1s2
1s22s22p6
16
Electrostatic (Lattice) Energy
Lattice energy (E) is the energy required to
completely separate one mole of a solid ionic
compound into gaseous ions.
Q is the charge on the cation
Q- is the charge on the anion
r is the distance between the ions
Lattice energy (E) increases as Q increases
and/or as r decreases.
r F lt r Cl
17
Covalent Bond
A chemical bond in which two or more electrons
are shared by two atoms.
Lewis structure of F2
18
Distribution of electron density of H2
H
H
19
Lewis structure of water


Double bond two atoms share two pairs of
electrons
or
Triple bond two atoms share three pairs of
electrons
or
20
Polar Covalent Bond
A covalent bond with greater electron density
around one of the two atoms
electron rich region
electron poor region
e- rich
e- poor
d
d-
21
Electron density distributions in H2, F2, and HF.
22
Electronegativities (EN)
The ability of an atom in a molecule to attract
shared electrons to itself
23
Classification of Bonds
Difference in EN
Bond Type
0
Covalent
? 2
Ionic
0 lt and lt2
Polar Covalent
24
Classification of Bonds
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
25
Rules for Writing Lewis Structures

1. Draw skeletal structure of compound showing what
   atoms are bonded to each other. Put least
   electronegative element in the center.
2. Count total number of valence e-. Add 1 for each
   negative charge. Subtract 1 for each positive
   charge.
3. Use one pair of electrons to form a bond (a
   single line) between each pair of atoms.
4. Arrange the remaining electrons to satisfy an
   octet for all atoms (duet for H), starting from
   outer atoms.
5. If a central atom does not have an octet, move in
   lone pairs to form double or triple bonds on the
   central atom as needed.

26
Step 1 N is less electronegative than F, put N
in center
Step 2 Count valence electrons N - 5 (2s22p3)
and F - 7 (2s22p5)
5 (3 x 7) 26 valence electrons
Step 3 Draw single bonds between N and F atoms.
Step 4 Arrange remaining 20 electrons to
complete octets
27
Step 1 C is less electronegative than O, put C
in center
Step 2 Count valence electrons C - 4 (2s22p2)
and O - 6 (2s22p4) -2 charge 2e-
4 (3 x 6) 2 24 valence electrons
Step 3 Draw single bonds between C and O atoms
Step 4 - Arrange remaining 18 electrons to
complete octets
Step 5 The central C has only 6 electrons. Form
a double bond.
28
Resonance
More than one valid Lewis structures can be
written for a particular molecule The actual
structure of the carbonate ion is an average of
the three resonance structures
29
Exceptions to the Octet Rule
The Incomplete Octet
BeH2
BF3
30
Exceptions to the Octet Rule
Odd-Electron Molecules
NO
The Expanded Octet (central atom with principal
quantum number n gt 2)
SF6
31
Covalent Bond Lengths
Bond Type Bond Length (pm)
C-C 154
C?C 133
C?C 120
C-N 143
C?N 138
C?N 116
Bond Lengths Triple bond lt Double Bond lt Single
Bond
32
Covalent Bond Energy
The energy required to break a particular bond in
one mole of gaseous molecules is the bond energy.
Bond Energy
33
Light-Matter Interactions
3 x 1020
3 x 1016
Frequency in Hz
Dissociation Ionization
Vibration
Rotation
34
Vibrational Modes of Water
35
Infrared Spectrum of Water
Liquid
Gas
Reveal the interactions between molecules and
their environments
36
Infrared Spectrum of Caffeine
Absorbance
2000
1000
3000
Wavenumber (cm-1)
Identification of compounds
37
Lab 1
38
Acknowledgment
Some images, animation, and material have been
taken from the following sources Chemistry,
Zumdahl, Steven S. Zumdahl, Susan A. Houghton
Mifflin Co., 6th Ed., 2003 supplements for the
instructor General Chemistry The Essential
Concepts, Chang, Raymon McGraw-Hill Co. Inc.,
4th Ed., 2005 supplements for the
instructor Principles of General Chemistry,
Silberberg, Martin McGraw-Hill Co. Inc., 1st
Ed., 2006 supplements for the instructor NIST
WebBook http//webbook.nist.gov/ http//www.lsbu.
ac.uk/water/vibrat.html http//en.wikipedia.org/wi
ki/Caffeine http//www.wilsonhs.com/SCIENCE/CHEMIS
TRY/MRWILSON/Unit20420Chemical2 0Bonding20Powe
rpoint1.ppt