Bonding

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 -.

HyperChem

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.

HyperChem

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

HyperChem

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-

HyperChem

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

HyperChem

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

HyperChem

Figure 9.12

Figure 9.11: Molecular Shape and Molecular Polarity

HyperChem

Figure 9.13: Molecular Shape and Molecular Polarity

HyperChem

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