chemical bonding
DESCRIPTION
Chemical Bonding. Chemical Bonds, Lewis Symbols, and the Octet Rule. Chemical bond : attractive force holding two or more atoms together. Covalent bond results from sharing electrons between the atoms. Usually found between nonmetals. - PowerPoint PPT PresentationTRANSCRIPT
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Ionic Bonding
Resonance Structures
VSEPRBasic Shapes3-D NotationHybridization (Lab)
M olecular G eom etries
Octet Rule Polar M olecules
Lew is Structures Covalent Bonding
T ypes of Bonds
Chemical BondingChemical Bonding
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• Chemical bond: attractive force holding two or more atoms together.
• Covalent bond results from sharing electrons between the atoms. Usually found between nonmetals.
• Ionic bond results from the transfer of electrons from a metal to a nonmetal.
• Metallic bond: attractive force holding pure metals together.
Chemical Bonds, Lewis Symbols, and the Octet RuleChemical Bonds, Lewis Symbols, and the Octet Rule
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Figure 8.3: Ionic BondingFigure 8.3: Ionic Bonding
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Figure 8.5: Covalent BondingFigure 8.5: Covalent Bonding
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Chemical BondsChemical Bonds
Bond Type Single Double Triple
# of e’s 2 4 6
Notation — =
Bond order 1 2 3
Bond strength
Increases from Single to Triple
Bond length Decreases from Single to Triple
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Strengths of Covalent BondsStrengths of Covalent Bonds
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Lewis Symbols
Chemical Bonds, Lewis Symbols, and the Octet RuleChemical Bonds, Lewis Symbols, and the Octet Rule
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The Octet Rule• All noble gases except He has an s2p6 configuration. • Octet rule: atoms tend to gain, lose, or share electrons
until they are surrounded by 8 valence electrons (4 electron pairs).
• Caution: there are many exceptions to the octet rule.
Chemical Bonds, Lewis Symbols, and the Octet RuleChemical Bonds, Lewis Symbols, and the Octet Rule
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Bond Polarity and ElectronegativityBond Polarity and Electronegativity
Electronegativity• Electronegativity: The ability of one atoms in a
molecule to attract electrons to itself.• Pauling set electronegativities on a scale from 0.7 (Cs) to
4.0 (F).• Electronegativity increases
• across a period and
• down a group.
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Figure 8.6: Electronegativities of ElementsFigure 8.6: Electronegativities of Elements
Electronegativity
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Bond Polarity and ElectronegativityBond Polarity and Electronegativity
Figure 8.7: Electronegativity and Bond Polarity• There is no sharp distinction between bonding types.• The positive end (or pole) in a polar bond is represented
+ and the negative pole -.
HyperChem
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Drawing Lewis StructuresDrawing Lewis Structures
Follow Step by Step Method (See Ng Web-site)
1. Total all valence electrons. [Consider Charge]
2. Write symbols for the atoms and guess skeleton structure [ define a central atom ].
3. Place a pair of electrons in each bond.
4. Complete octets of surrounding atoms. [ H = 2 only ]
5. Place leftover electrons in pairs on the central atom.
6. If there are not enough electrons to give the central atom an octet, look for multiple bonds by transferring electrons until each atom has eight electrons around it.
HyperChemCyberChem video
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Lewis Structures - ExamplesLewis Structures - Examples
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Exceptions to the Octet RuleExceptions to the Octet Rule
Less than an Octet• Relatively rare.• Molecules with less than an octet are typical for
compounds of Groups 1A, 2A, and 3A.
• Most typical example is BF3.
• Formal charges indicate that the Lewis structure with an incomplete octet is more important than the ones with double bonds.
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Exceptions – Central Atoms - Less than an OctetExceptions – Central Atoms - Less than an Octet
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Exceptions to the Octet RuleExceptions to the Octet Rule
More than an Octet• This is the largest class of exceptions.• Atoms from the 3rd period onwards can accommodate
more than an octet.• Beyond the third period, the d-orbitals are low enough in
energy to participate in bonding and accept the extra electron density.
HyperChem
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Exceptions – Central Atoms - Greater than an OctetExceptions – Central Atoms - Greater than an Octet
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• There are five fundamental geometries for molecular shape:
Molecular ShapesMolecular Shapes
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Figure 9.3
HyperChem
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Molecular Shapes – 3D NotationsMolecular Shapes – 3D Notations
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Summary of VSEPR Molecular ShapesSummary of VSEPR Molecular Shapese-pairs Notation Name of VSEPR shape Examples
2 AX2 Linear HgCl2 , ZnI2 , CS2 , CO2
3 AX3 Trigonal planar BF3 , GaI3
AX2E Non-linear (Bent) SO2 , SnCl2
4 AX4 Tetrahedral CCl4 , CH4 , BF4-
AX3E (Trigonal) Pyramidal NH3 , OH3-
AX2E2 Non-Linear (Bent) H2O , SeCl2
5 AX5 Trigonal bipyramidal PCl5 , PF5
AX4E Distorted tetrahedral
(see-sawed)
TeCl4 , SF4
AX3E2 T-Shaped ClF3 , BrF3
AX2E3 Linear I3- , ICl2
-
6 AX6 Octahedral SF6 , PF6-
AX5E Square Pyramidal IF5 , BrF5
AX4E2 Square Planar ICl4- , BrF4
-See Ng Web-siteHyperChem CyberChem video
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Examples: VSEPR Molecular ShapesExamples: VSEPR Molecular Shapes
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The Effect of Nonbonding Electrons
• By experiment, the H-X-H bond angle decreases on moving from C to N to O:
• Since electrons in a bond are attracted by two nuclei, they do not repel as much as lone pairs.
• Therefore, the bond angle decreases as the number of lone pairs increase
VSEPR ModelVSEPR Model
104.5O107O
NHH
HC
H
HHH109.5O
OHH
HyperChem
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Figure 9.10: Shapes of Larger Molecules
• In acetic acid, CH3COOH, there are three central atoms.
VSEPR ModelVSEPR ModelHyperChem
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Figure 8.10: Drawing Lewis StructuresFigure 8.10: Drawing Lewis Structures
Resonance Structures
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Figure 9.12
HyperChem
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Figure 9.11: Molecular Shape and Molecular PolarityFigure 9.11: Molecular Shape and Molecular Polarity
HyperChem
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Figure 9.13: Molecular Shape and Molecular PolarityFigure 9.13: Molecular Shape and Molecular Polarity
HyperChem
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• Lewis structures and VSEPR do not explain why a bond forms.
• How do we account for shape in terms of quantum mechanics?
• What are the orbitals that are involved in bonding?• We use Valence Bond Theory:
• Bonds form when orbitals on atoms overlap.
• There are two electrons of opposite spin in the orbital overlap.
Covalent Bonding and Orbital OverlapCovalent Bonding and Orbital Overlap
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Figure 9.14: Covalent Bonding and Orbital OverlapFigure 9.14: Covalent Bonding and Orbital Overlap
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• To determine the electron pair geometry:• draw the Lewis structure,
• count the total number of electron pairs around the central atom,
• arrange the electron pairs in one of the above geometries to minimize e-e repulsion, and count multiple bonds as one bonding pair.
VSEPR Model (Figure 9.6)VSEPR Model (Figure 9.6)
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VSEPR VSEPR ModelModel
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Drawing Lewis StructuresDrawing Lewis Structures
Formal Charge• Consider:
• For C: • There are 4 valence electrons (from periodic table).
• In the Lewis structure there are 2 nonbonding electrons and 3 from the triple bond. There are 5 electrons from the Lewis structure.
• Formal charge: 4 - 5 = -1.
C N
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Drawing Lewis StructuresDrawing Lewis Structures
Formal Charge• Consider:
• For N:• There are 5 valence electrons.• In the Lewis structure there are 2 nonbonding electrons and 3
from the triple bond. There are 5 electrons from the Lewis structure.
• Formal charge = 5 - 5 = 0.• We write:
C N
C NCyberChem video
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Ionic Bonding
Resonance Structures
VSEPRBasic Shapes3-D NotationHybridization (Lab)
M olecular G eom etries
Octet Rule Polar M olecules
Lew is Structures Covalent Bonding
T ypes of Bonds
Chemical BondingChemical Bonding
Lewis
AXE notation
VSEPR shapes
Polarity