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Molecular Geometry

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Molecular Geometry
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Molecular Geometry

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  1. Molecular Geometry And Polarity

  2. 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?

  3. 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

  4. Octet Rule • Atoms form bonds to achieve noble gas configuration = 8 electrons in each atoms valence shell • Exception = H • Why?

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

  6. 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

  7. 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

  8. 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

  9. 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

  10. Example Multiple Bond • Ex: CO

  11. Example Polyatomic Anion • (PO4)3-

  12. Resonance • More than one correct Lewis Structure

  13. 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

  14. Molecular Shape Objectives • Discuss the VSEPR bonding theory • Predict the shape of and the bond angles in a molecule • Define hybridization

  15. 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

  16. 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

  17. Electronegativity

  18. Determine Bond Type • Diff. EN > 1.7 : Ionic • 0.1 < Diff. EN < 1.7 : Polar Covalent • Diff. EN < 0.1 : Nonpolar Covalent

  19. Determine Bond Type • H2 • H2O • KCl

  20. Polar Covalent Bonds • Atoms do not share bonded pair equally • More electronegative atom attracts the bonded pair more forming partial charges

  21. 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

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

  23. 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-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.)

  24. Determine whether the following molecules are polar or nonpolar • SCl2 • H2S • CF4 • CS2

  25. Water Molecule

  26. Properties of Molecular Compounds • Intermolecular Forces: • Force of attraction between molecules