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VSEPR

VSEPR. Valence Shell Electron Pair Repulsion Theory. Vocabulary:. “ domain ” = any electron pair, bond, or any double or triple bond is considered one domain. “ lone pair ” = “ non-bonding pair ” = “ unshared pair ” = any electron pair that is not involved in bonding.

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VSEPR

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  1. VSEPR Valence Shell Electron Pair Repulsion Theory

  2. Vocabulary: “domain” = any electron pair, bond, or any double or triple bond is considered one domain. “lone pair” = “non-bonding pair” = “unshared pair” = any electron pair that is not involved in bonding “bonding pair” = “shared pair” = any electron pair that is involved in bonding

  3. 2 things on central atom Example: BeCl2 1) Central Atom? Be (only 1 atom)

  4. Cl Be Cl VSEPR THEORY: Example: BeCl2 2) Electron Dot? 2) Bar Diagram? Note that Be violates the octet rule—this is an exception! Cl—Be—Cl

  5. Be Cl Cl VSEPR THEORY: Example: BeCl2 3) Geometry? Hint: What is the furthest apart you can spread two atoms attached to a central atom?

  6. Be Cl Cl VSEPR THEORY: Example: BeCl2 4) Shape? • LINEAR – 180o

  7. 2 things on central atom LINEAR • 2 bonds • both are bonding pairs • They push each other to opposite sides of center (180 apart). BeCl2

  8. 2 things on central atom • Does not matter if double or triple bonds! • Still 2 things on central atom • Examples: • ~ CO2 • ~ HCN

  9. Example: BF3 3 things on central atom 1) Central Atom? B (only 1 atom)

  10. F B F F VSEPR THEORY: Example: BF3 2) Electron Dot? 2) Bar Diagram? Note that B violates the octet rule—this is an exception! F—B—F F

  11. VSEPR THEORY: Example: BF3 3) Geometry? Hint: What is the furthest apart you can spread three atoms attached to a central atom? F B F F

  12. F B F F VSEPR THEORY: Example: BF3 4) Shape? • trigonal planar

  13. 3 things on central atom TRIGONAL PLANAR • 3 bonds • all are bonding pairs • They push each other apart equally at 120 degrees. BF3

  14. 3 things on central atom • Again double, triple bonds do not matter and do not change shape • Still three bonds to central atom • Example ~ CH2O

  15. 3 things on central atom BENT • 3 things: • 2 are bonding pairs • 1 is a lone pair • The 2 bonding pairs are pushed apart by 3rd pair (not seen) • Lone pair pushes little harder and bonds are < 120o (~116o) SnF2

  16. NOTE: • The geometry around the central atom is trigonal planar. • The molecular shape is bent. SnF2

  17. Example: CH4 4 things on central atom 1) Central Atom? C (only 1 atom)

  18. H H C H H H H—C—H H VSEPR THEORY: Example: CH4 2) Electron Dot? 2) Bar Diagram?

  19. H C H H H VSEPR THEORY: Example: CH4 3) Geometry? Hint: What is the furthest apart you can spread four atoms attached to a central atom? Think in 3D!

  20. H C H H H VSEPR THEORY: Example: CH4 4) Shape?  tetrahedral

  21. 4 things on central atom TETRAHEDRAL • 4 bonds • Each repels the other equally - 109.5 - not the expected 90. • Think in 3D. CH4

  22. Example: NH3 4 things on central atom 1) Central Atom? N (only 1 atom)

  23. H N H H H—N—H H VSEPR THEORY: Example: NH3 2) Electron Dot? 2) Bar Diagram?

  24. ~109.5o H N H H VSEPR THEORY: Example: NH3 3) Geometry? Hint: What is the furthest apart you can spread three atoms plus one unbonded pair of electrons attached to a central atom? Think in 3D!

  25. < 109.5 (~107o) H N H H VSEPR THEORY: Example: NH3 4) Shape?  trigonal pyramidal

  26. 4 things on central atom TRIGONAL PYRAMIDAL • 4 domains • 3 bonding pairs • 1 lone pair • The thicker, lone pair forces the others a little bit closer together (~107) NH3

  27. 4 domains on central atom BENT • 4 domains • 2 bonding pairs • 2 lone pairs • The bonds are forced together still closer (104.5) by the 2 thick unshared pairs. H2O

  28. Comparing the 2 “bents”… Both bent molecules are affected by unshared pairs – 1 pair on the left, 2 on the right.

  29. Other Molecular Geometry Just for fun, lets look at some others that we will not study in detail in this course… Note that if there are more than five domains around the central atom, it must be an exception to the octet rule!

  30. 5 e- pairs on central atom TRIGONAL BIPYRAMIDAL • 5 shared pairs • Three pairs are found in one plane (“equator”) 120 apart; the other two pairs are at the “poles,” 180 apart, 90 from the “equator.” PCl5

  31. 5 e- pairs on central atom SEE-SAW • 4 shared pairs & 1 unshared pair • One of the equator pairs is unshared & pushes the other 2 together. • The 2 poles are pushed slightly together. SF4

  32. 5 e- pairs on central atom T-SHAPED • 3 shared & 2 unshared pairs • 2 of the 3 equator pairs are unshared. • All 3 remaining pairs are pushed together. ClF3

  33. 5 e- pairs on central atom LINEAR • 2 shared & 3 unshared pairs • All 3 equator pairs are unshared. The 2 remaining pairs are forced to the poles. XeF2

  34. 5 e- pairs on central atom 4 shared, 1 unshared 5 shared, 0 unshared 3 shared, 2 unshared 2 shared, 3 unshared

  35. 6 e- pairs on central atom OCTAHEDRAL • 6 shared pairs • Each pair repels the others equally. • All angles = 90 Now, if one of these pairs was unshared … SF6

  36. 6 e- pairs on central atom SQUARE PYRAMIDAL • 5 shared pairs & 1 unshared pair • 4 shared pairs in one plane; the 5th pair at the pyramid’s top. If the pair at the top was unshared … IF5

  37. 6 e- pairs on central atom SQUARE PLANAR • 4 shared & 2 unshared pairs • The 4 shared pairs are in the same plane; the 2 unshared pairs are 90 from them. XeF4

  38. 6 e- pairs on central atom 6 shared, 0 unshared 5 shared, 1 unshared 4 shared, 2 unshared

  39. Steps for using VSEPR: • Draw a Lewis Dot Structure. • Predict the geometry around the central atom. • Predict the molecular shape. … also, we can try and predict the angles between atoms.

  40. All e- pairs push each other as far apart as possible. • Shared (bonding) pairs are “stretched” between two atoms that want them. • “Longer & Thinner” • Unshared (non-bonding) pairs are not “stretched.” • “Shorter & Thicker”

  41. Electron Pair Repulsion • 2 lone pairs require the most space & repel each other the most, resulting in the greatest distance (angle). • 1 lone pair (thick) & 1 bonding pair (thin) require less space • 2 bonding pairs (both thin) require the least space & repel each other the smallest distance (angle).

  42. Let try some… • How many things around central atom? • What structure is the molecule based on? • Are any atoms replaced by lone electron pairs? • What is the final structure? Bond angles? • PF3 • NO2- • NO3- • OF2 • CO2 • NH4+

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