Molecular Geometry

And Polarity

Essential Questions

How do properties of polar and nonpolar bonds and molecules compare?

How would you describe the structure of the water molecule and how does this structure affect its function?

How do the properties of water depend on the arrangement of the atoms in the molecule?

What are the unique properties of water?

ObjectivesDraw lewis structures of molecules

Explain why resonance occurs and identify resonance structures

Explain three exceptions to the octet rule and identify molecules in which these exceptions occur

Octet Rule

Atoms form bonds to achieve noble gas configuration = 8 electrons in each atoms valence shell

Exception = HWhy?

Structural Formulas/

Lewis StructuresShows relative position of atoms as well as number of bonds

Procedures1. Add up the total number of valence electrons for all the atoms. Account for charge: If a species has a negative charge (-) add one valence electron for each charge. If a species has a positive charge (+) subtract one electron for each positive charge

Ex.: PH3 1xP = 5 e- 3xH = 3 e-

Total: 8 valence electrons

Procedures2. Draw the molecular skeleton and connect the atoms with one bond. The central atom is generally the atom with the lowest electronegativity, but never H

PH3

Procedure3. Satisfy the octet rule. Distribute the remaining valence electrons by adding lone pairs to complete the octets of the outer atoms first (H only requires two valence electrons), then place any remaining electrons on the central atom.

PH3

Procedure4. If there are too few valence electrons to give each atom an octet, multiple bonds may be required. In this case convert outer atom lone pairs to bond pairs to form multiple bonds

Example Multiple Bond

Ex: CO

Example Polyatomic Anion(PO4)3-

ResonanceMore than one correct Lewis Structure

Exceptions to Octet

Only a Rule of Thumb

Atoms without octetsBF3

Could satisfy with double bond but experimental evidence shows only single bonds which takes precedence

Atoms with more than an octetXeF4

PCl5

These are possible because of bonding d orbitals

Called Expanded Octet

Molecular Shape Objectives

Discuss the VSEPR bonding theory

Predict the shape of and the bond angles in a molecule

Define hybridization

Molecular ShapeThe shape of the molecule effects how it reacts with other molecules

Use VSEPR model – Valence Shell Electron Repulsion model

Electron pairs repel each other resulting in fixed bond angles

Lone electron pairs - occupy larger orbitals – bonded pairs pushed together slightly by lone pairs

Electronegativity and Polarity ObjectivesDescribe how electronegativity is used

to determine bond type

Compare and contrast polar and nonpolar covalent bonds and polar and nonpolar molecules

Describe the characteristics of compounds that are covalently bonded and compare and contrast them to ionic compounds

Electronegativity

Determine Bond Type

Diff. EN > 1.7 : Ionic

0.1 < Diff. EN < 1.7 : Polar Covalent

Diff. EN < 0.1 : Nonpolar Covalent

Determine Bond Type

H2

H2O

KCl

Polar Covalent Bonds

Atoms do not share bonded pair equally

More electronegative atom attracts the bonded pair more forming partial charges

Dipole MomentDipoles form when the bondBetween two atoms is polar.Dipoles may or may notResult in polar molecules. Polar molecules align theirPolarity in electric fields

Polar MoleculesMolecules can be either polar or nonpolar

H2O and CCl4

Polar molecule

Nonpolar molecule

Polarity RulesDetermining Polarity

Is it polar? There are three ways to go about determining whether a molecule is polar or not.

A. If the molecule has a net dipole, then it is polar.B. If the structure is symmetric, then it is non-polarC. There are three rules to this part:1. When there are no lone pairs on the center atom, then the molecule is non-polar2. If it is linear or square planar, then it is non-polar. (This rule is more important than rule 1, so it overrules it because it has lone pairs.)3. If it has different terminal atoms, then it is polar. (This rule overrules rule 1 and 2 because it is more important.)

Determine whether the following molecules are polar or nonpolar

SCl2

H2S

CF4

CS2

Water Molecule

Properties of Molecular

CompoundsIntermolecular Forces:Force of attraction between molecules