bonding
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Chemical BondingTypes of Bonds
Ionic Bonding
Lewis Structures
Covalent Bonding
Resonance Structures
Octet Rule
Polar Molecules
Molecular Geometries VSEPR Basic Shapes 3-D Notation Hybridization (Lab)
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. Ionic bond results from the transfer of electrons from a metal to a nonmetal. Metallic bond: attractive force holding pure metals together.
Figure 8.3: Ionic Bonding
Figure 8.5: Covalent Bonding
Chemical BondsBond Type# of es Notation Bond order
Single2 1
Double4 = 2
Triple6 3
Bond strength Bond length
Increases from Single to Triple Decreases from Single to Triple
Strengths of Covalent Bonds
Chemical Bonds, Lewis Symbols, and the Octet Rule
Lewis Symbols
Chemical Bonds, Lewis Symbols, and the Octet Rule
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.
Bond Polarity and ElectronegativityElectronegativity 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.
Figure 8.6: Electronegativities of ElementsElectronegativity
Bond Polarity and ElectronegativityFigure 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 -.
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Drawing Lewis Structures1. 2. 3. 4. 5. 6. Follow Step by Step Method (See Ng Web-site) Total all valence electrons. [Consider Charge] Write symbols for the atoms and guess skeleton structure [ define a central atom ]. Place a pair of electrons in each bond. Complete octets of surrounding atoms. [ H = 2 only ] Place leftover electrons in pairs on the central atom. 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.HyperChem
CyberChem (Lewis) video
Lewis Structures Examples - I
Lewis Structures Examples I F12
Lewis Structures Examples - II
Lewis Structures Examples II F12
Lewis Structures Examples - III
Lewis Structures Examples - IV
Exceptions to the Octet Rule Central Atoms Having 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.
Exceptions Central Atoms - Less than an Octet
Exceptions to the Octet RuleCentral Atoms Having 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.
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Exceptions Central Atoms - Greater than an Octet
Molecular Shapes: VSEPR
There are five fundamental geometries for molecular shape:
Molecular Shapes 3D Notations
VSEPR (Ballons)-Movie Clip
Figure 9.3
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Summary of VSEPR Molecular Shapese-pairs2 3 4
NotationAX2 AX3 AX2E AX4 AX3E AX2E2
Name of VSEPR shape ExamplesLinear Trigonal planar Non-linear (Bent) Tetrahedral (Trigonal) Pyramidal Non-Linear (Bent) Trigonal bipyramidal Distorted tetrahedral (see-sawed) T-Shaped Linear Octahedral HgCl2 , ZnI2 , CS2 , CO2 BF3 , GaI3 SO2 , SnCl2 CCl4 , CH4 , BF4NH3 , OH3H2O , SeCl2 PCl5 , PF5 TeCl4 , SF4 ClF3 , BrF3 I3- , ICl2SF6 , PF6-
5
AX5 AX4E AX3E2 AX2E3
6
AX6
AX5EAX4E2
Square PyramidalSquare Planar
IF5 , BrF5ICl4- , BrF4-
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CyberChm Gems
See Ng Web-site
Examples: VSEPR Molecular Shapes - I# electron pairs on Central Atom A Notation Example Lewis VSEPR & Name of Shape
2
AX2 2 bp on A
AX3 3 bp on A 3 AX2E 2 bp and 1 lp on A
Examples: VSEPR Molecular Shapes I F08
Examples: VSEPR Molecular Shapes - II# electron pairs on Central Atom A Notation Example Lewis VSEPR & Name of Shape
AX4 4 bp on A
4
AX3E 3 bp and 1 lp on A
AX2E2 2 bp and 2 lp on A
Examples: VSEPR Molecular Shapes II F08
Examples: VSEPR Molecular Shapes - III# electron pairs on Central Atom A Notation Example Lewis VSEPR & Name of Shape
AX5 5 bp on A AX4E 4 bp and 1 lp on A 5 AX3E2 3 bp and 2 lp on A AX2E3 2 bp and 3 lp on A
Examples: VSEPR Molecular Shapes III F08
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Examples: VSEPR Molecular Shapes - IV# electron pairs on Central Atom A Notation Example Lewis VSEPR & Name of Shape
AX6 6 bp on A
6
AX5E 5 bp and 1 lp on A
AX4E2 4 bp and 2 lp on A
Examples: VSEPR Molecular Shapes - IV# electron pairs on Central Atom A Notation Example Lewis VSEPR & Name of Shape
AX6 6 bp on A
6
AX5E 5 bp and 1 lp on A
AX4E2 4 bp and 2 lp on A
VSEPR ModelThe Effect of Nonbonding Electrons
By experiment, the H-X-H bond angle decreases on moving from C to N to O:H H C H H 109.5O H N H H 107O H H 104.5O 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 increases HyperChem
VSEPR ModelHyperChem
Figure 9.10: Shapes of Larger Molecules In acetic acid, CH3COOH, there are three central atoms.
Lewis-VSEPR HW assigned 10/28/11 . Due 10/31/11.HyperChemShapes of Larger Molecules In glycine (simplest amino acid), NH2CH2CO2H, there are four possible central atoms. Draw the Lewis Structure and the 3D VSEPR Molecular Geometry for glycine. Indicate the name of the shape for all possible central atoms, including estimation of bond angles. Hint 1: Designate the 2nd carbon in the formula as the central atom in skeleton structure. Hint 2: The acid portion of glycine is the same as that of acetic acid.
Solution Key
Figure 8.10: Drawing Lewis Structures
Resonance Structures
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Figure 9.12
Figure 9.11: Molecular Shape and Molecular Polarity
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Figure 9.13: Molecular Shape and Molecular Polarity
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Covalent Bonding and Orbital OverlapGems - Movie Clip
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.
Figure 9.14: Covalent Bonding and Orbital Overlap
VSEPR Model (Figure 9.6)
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
Drawing Lewis StructuresFormal Charge
Consider: For C:
C N
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.
Drawing Lewis StructuresFormal Charge
Consider: For N:
C 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 NCyberChm Gems
Chemical BondingTypes of Bonds
Ionic Bonding
Lewis Structures
Covalent Bonding
Resonance Structures
Octet Rule
Polar Molecules
Lewis
Molecular Geometries VSEPR Basic Shapes 3-D Notation Hybridization (Lab)
VSEPR shapes
AXE notation
Polarity
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