molecular geometry and polarity. essential questions how do properties of polar and nonpolar bonds...
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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