# Molecular Geometry And Polarity.

## Presentation on theme: "Molecular Geometry And Polarity."— Presentation transcript:

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?

Objectives Draw 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 = H Why?

Structural Formulas/ Lewis Structures
Shows relative position of atoms as well as number of bonds

Procedures 1. 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

Procedures 2. 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

Procedure 3. 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

Procedure 4. 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

Resonance More than one correct Lewis Structure

Exceptions to Octet Only a Rule of Thumb Atoms without octets BF3
Could satisfy with double bond but experimental evidence shows only single bonds which takes precedence Atoms with more than an octet XeF4 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 Shape The 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 Objectives
Describe 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 Moment Dipoles form when the bond Between two atoms is polar.
Dipoles may or may not Result in polar molecules. Polar molecules align their Polarity in electric fields

Polar Molecules Molecules can be either polar or nonpolar H2O and CCl4
Nonpolar molecule

Polarity Rules Determining 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-polar C. There are three rules to this part: 1. When there are no lone pairs on the center atom, then the molecule is non-polar 2. 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 Compounds
Intermolecular Forces: Force of attraction between molecules