1 When Atoms Meet. 2 Bonding Forces Electron – electron repulsive forces Nucleus – nucleus...
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Transcript of 1 When Atoms Meet. 2 Bonding Forces Electron – electron repulsive forces Nucleus – nucleus...
1
When Atoms Meet
2
Bonding Forces
Electron – electron repulsive forces
Nucleus – nucleus repulsive forces
Electron – necleus attractive forces
Bonds Forces that hold groups of atoms
together and make them function
as a unit.
3
Metals and Nonmetals
4
Types of Chemical Bonding
1. Metal with nonmetal:
electron transfer and ionic bonding
5
Three models of chemical bonding
Electron transfer
Ionic
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
Electron transfer Electron sharing
Ionic Covalent
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
Electron transfer Electron sharing Electron pooling
Ionic Covalent Metallic
10
9.1
• The outer shell electrons of an atom• Participate in chemical bonding
1A 1ns1
2A 2ns2
3A 3ns2np1
4A 4ns2np2
5A 5ns2np3
6A 6ns2np4
7A 7ns2np5
Group # of valence e-e- configuration
Valence Electrons
11
G. N. Lewis
Developed the idea in 1902.
Lewis Structures
12
Nitrogen, N, is in Group 5A and therefore has 5 valence electrons.
N:.
..
:
N .. ..N :.
. :N ...
Place one dot per valence electron on each of the four sides of the element symbol.
Pair the dots (electrons) until all of the valence electrons are used.
Lewis Dot Symbols
13
Lewis Dot Symbols
14
The Octet RuleChemical 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
Li + F Li+ F -
Ionic Bond
1s22s11s22s22p5 1s21s22s22p6[He][Ne]
Li
1s 2s 2p
F
1s 2s 2p
+
Li+
1s 2s 2p
F-
1s 2s 2p+
16
Lattice energy (E) increases as Q increases and/or
as r decreases.
cmpd lattice energyMgF2
MgO
LiF
LiCl
2957
3938
1036
853
Q= +2,-1
Q= +2,-2
r F < r Cl
Electrostatic (Lattice) Energy
E = kQ+Q-r
Q+ is the charge on the cation
Q- is the charge on the anionr is the distance between the ions
Lattice energy (E) is the energy required to completely separate one mole of a solid ionic compound into gaseous ions.
17
A chemical bond in which two or more electrons are shared by two atoms.
How should two atoms share electrons?
F F+
7e- 7e-
F F
8e- 8e-
F F
F F
Lewis structure of F2
lone pairslone pairs
lone pairslone pairs
single covalent bond
single covalent bond
Covalent Bond
18
Distribution of electron density of H2
H H
19
8e-
H HO+ + OH H O HHor
2e- 2e-
Lewis structure of water
Double bond – two atoms share two pairs of electrons
single covalent bonds
O C O or O C O
8e- 8e-8e-double bonds double bonds
Triple bond – two atoms share three pairs of electrons
N N8e-8e-
N N
triple bondtriple bond
or
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H F FH
A covalent bond with greater electron density around one of the two atoms
electron richregion
electron poorregion e- riche- poor
+ -
Polar Covalent Bond
21
Electron density distributions in
H2, F2, and HF.
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Electronegativities (EN)
The ability of an atom in a molecule to attract shared electrons to itself
Linus Pauling 1901 - 1994
23
Covalent
share e-
Polar Covalent
partial transfer of e-
Ionic
transfer e-
Increasing difference in electronegativity
Classification of Bonds
Difference in EN Bond Type
0 Covalent
2 Ionic
0 < and <2 Polar Covalent
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Classify the following bonds as ionic, polar covalent, or covalent: The bond in CsCl; the bond in H2S; andthe 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
Classification of Bonds
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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.
Rules for Writing Lewis Structures
26
Write the Lewis structure of nitrogen trifluoride (NF3).
Step 1 – N is less electronegative than F, put N in center
F N F
F
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
Write the Lewis structure of the carbonate ion (CO32-).
Step 1 – C is less electronegative than O, put C in center
O C O
O
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.
2
28
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
O C O
O
- -O C O
O
-
-
OCO
O
-
-
Resonance
2 2 2
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Exceptions to the Octet Rule
The Incomplete Octet
H HBeBe – 2e-
2H – 2x1e-
4e-
BeH2
BF3
B – 3e-
3F – 3x7e-
24e-
F B F
F
3 single bonds (3x2) = 69 lone pairs (9x2) = 18
Total = 24
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Exceptions to the Octet Rule
Odd-Electron Molecules
N – 5e-
O – 6e-
11e-
NO N O
The Expanded Octet (central atom with principal quantum number n > 2)
SF6
S – 6e-
6F – 42e-
48e-
S
F
F
F
FF
F
6 single bonds (6x2) = 1218 lone pairs (18x2) = 36
Total = 48
31
Bond Type
Bond Length
(pm)
C-C 154
CC 133
CC 120
C-N 143
CN 138
CN 116
Covalent Bond Lengths
Bond Lengths
Triple bond < Double Bond < Single Bond
32
The energy required to break a particular bond in one mole of gaseous molecules is the bond energy.
H2 (g) H (g) + H (g) 436.4 kJ
Cl2 (g) Cl (g)+ Cl (g) 242.7 kJ
HCl (g) H (g) + Cl (g) 431.9 kJ
O2 (g) O (g) + O (g) 498.7 kJ O O
N2 (g) N (g) + N (g) 941.4 kJ N N
Bond Energy
Bond Energies
Single bond < Double bond < Triple bond
Covalent Bond Energy
33
Frequency in Hz
3 x 1020 3 x 1016 3 x 1012 3 x 108 3 x 104
DissociationIonization
Vibration
Rotation
Light-Matter Interactions
34
Vibrational Modes of WaterInfrared light
35
Infrared Spectrum of Water
100015002000
Wavenumber (cm-1)
Abs
orba
nce
100020003000
Liquid
Gas
Reveal the interactions between molecules and their environments
36
Infrared Spectrum of Caffeine
100020003000
Wavenumber (cm-1)
Abs
orba
nce
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/wiki/Caffeine
http://www.wilsonhs.com/SCIENCE/CHEMISTRY/MRWILSON/Unit%204%20Chemical%2
0Bonding%20Powerpoint1.ppt