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Unit 7 Molecular Geometry

Unit 7 Molecular Geometry. Essential Question. What is a Lewis structure and what does it tell us?. Lewis Structures. Properties are determined by molecular shape Lewis structures can be used to determine shape Shows the arrangement of electrons in a compound. Writing Lewis Structures.

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Unit 7 Molecular Geometry

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  1. Unit 7Molecular Geometry

  2. Essential Question • What is a Lewis structure and what does it tell us?

  3. Lewis Structures • Properties are determined by molecular shape • Lewis structures can be used to determine shape • Shows the arrangement of electrons in a compound

  4. Writing Lewis Structures • Step 1: Add up valence electrons • Ex: H2O H = 1 valence electron X 2 atoms O = 6 valence electrons 8 valence electrons

  5. Writing Lewis Structures • Step 2: Determine the central atom • If Carbon is in the compound, it is the central atom • If Carbon is NOT present, the LEAST electronegative element is central • Hydrogen is NEVER the central atom

  6. Writing Lewis Structures • Step 3: Draw a pair of electrons between the central atom and each bonded atom • A pair of bonding electrons can be represented by a dash or two dots between the bonding atoms H : O : H or H – O – H

  7. Writing Lewis Structures • Step 4: Fill in the rest of the electrons to satisfy the octet rule for all elements in the compound • Hydrogen must satisfy the duet rule .. .. H : O : H or H – O – H .. ..

  8. Writing Lewis Structures • Step 5: Count up all the electrons. Make sure everything has an octet (duet for hydrogen). • If you always check your work, you can’t make a mistake!! • HINT: For these instructions and more helpful tips see page 22 in your packet 

  9. Writing Lewis Structures • If necessary, use two pairs of electrons to form a double bond between two atoms • Ex: CO2

  10. Writing Lewis Structures • If necessary, use three pairs of electrons to form a triple bond between two atoms • Ex: N2

  11. Essential Question • What is polarity?

  12. Polarity • Polar bonds are an intermediate between covalent bonds (equal sharing) and ionic bonds (complete transfer of electrons)

  13. Polarity and Electronegativity • Recall from previous units… • Electronegativity is the ability of an element to attract elements in a compound • High = very attracted to electrons • Low = not so attracted to electrons • Some values can be located on page pg 21 in your packet.

  14. Polarity and Electronegativity • You can determine if a bond is nonpolar covalent, polar, covalent or ionic using electronegativity values (see pg 21 in your packet) • How big is the difference? 3.3 1.7 0.3 0.0 Ionic Polar Covalent Nonpolar Covalent

  15. Now we can determine Lewis structure for a molecule and polarity for a bond • Weren’t we looking for shape??

  16. Essential Questions • What is molecular geometry? • What determines basic molecular geometry?

  17. VSEPR • Valence • Shell • Electron • Pair • Repulsion

  18. VSEPR • VSEPR theory determines the molecular geometry of the molecule • Molecular Geometry – 3D arrangement of a molecule’s atoms in space • Number and type of electron pairs determines shape

  19. VSEPR • We can classify as bonding pairs or lone pairs • Bonding Pair – shared between two atoms • Lone Pair – unshared electrons • Number of each type of electron pair determines shape

  20. VSEPR • Molecular geometry determines the bond angle • Electron pairs avoid each other, so they will stay as far apart as possible • See page 21 in your packet

  21. VSEPR (reference pg 17 or 21 in your packet)

  22. VSEPR • To use determine the shape: • Draw Lewis Structure • Either A - Count the number of lone and bonding pairs, and compare to the chart on the previous slide (this would require you to MEMORIZE the chart) or B - Compare to the chart on page 21 in your packet (that’s what you get on the test)

  23. Molecular Polarity • Some molecules that have polar bonds are nonpolar • Molecular shapes can cause polarity to cancel out

  24. Molecular Polarity • Polar molecules are said to have dipoles • The dipoles tell you which parts of the molecule are slightly negative and slightly positive

  25. Molecular Polarity • For BINARY Compounds • Nonpolar Shapes • Tetrahedral • Linear • Trigonal Planar • Polar Shapes • Pyramidal • Bent

  26. Molecular Polarity • Ternary Compounds will generally be polar • Different elements have different electronegativities, so when there are more than two types of atoms the molecule will not be symetrical

  27. What Does All This Mean?!? • Polarity determines several properties

  28. “Like Dissolves Like” • Polar substances can be dissolved by other polar substances • Nonpolar substances can be dissolved by other nonpolar substances • Polar substances will NOT dissolve in nonpolar substances

  29. “Like Dissolves Like” • When two liquids are capable of mixing, they are miscible • Immiscible means that they will not mix

  30. “Like Dissolves Like” • Water (H2O) is a POLAR substance • So is glass (SiO2) • When water is in a glass container, why does the meniscus curve up?

  31. “Like Dissolves Like” • We know glass is POLAR • Hexane is NONPOLAR • Explain the behavior of this meniscus

  32. “Like Dissolves Like” • Is density the only reason why your salad dressing separates??

  33. More Properties

  34. More Properties

  35. What type of bear dissolves in water??

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