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Chemistry 20 - Chapter 3

Chemistry 20 - Chapter 3. 3.3 Molecular Shapes. VSEPR Theory. VSEPR stands for Valence Shell Electron Pair Repulsion.

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Chemistry 20 - Chapter 3

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  1. Chemistry 20 - Chapter 3 3.3 Molecular Shapes

  2. VSEPR Theory • VSEPR stands for Valence Shell Electron Pair Repulsion. • The whole concept revolves around the idea that the electrons in a molecule repel each other and will try and get as far away from each other as possible. VSEPR explains a lot about molecular geometry and structure, BUT NOT EVERYTHING!!

  3. VSEPR Theory • The electrons (both in pairs and singles as you will see) are "attached" to a central atom in the molecule and can "pivot" freely on the atom's surface to move away from the other electrons.

  4. Electrons will come in several flavors: • a) bonding pairs - this set of two electrons is involved in a bond, so we will write the two dots BETWEEN two atoms. This applies to single, double, and triple bonds. • b) nonbonding pairs - this should be rather obvious. • c) single electrons - in almost every case, this single electron will be nonbonding.

  5. VSEPR uses a set of letters to represent general formulas of compounds. These are: • a) A - this is the central atom of the molecule (or portion of a large molecule being focused on). • b) X - this letter represents the ligands or atoms attached to the central atom. No distinction is made between atoms of different elements. For example, AX4 can refer to CH4 or to CCl4.

  6. c) E - this stands for nonbonding electron pairs.d) e - this stands for lone nonbonding electrons.

  7. VSEPR shapes • Linear • Angular • Trigonal Planar • Trigonal Pyramidal • Tetrahedaral

  8. AX2 – Linear • Example: BeH2 • Lewis Dot: H:Be:H • Stereochemical Formula H-----Be-----H

  9. AX2E – Angular • Example: BrNO • Lewis Dot • Stereochemical Formula

  10. AX3 - Trigonal planar • Example: BF3 • Stereochemical Formula

  11. AX4 - Tetrahedral • Example: CCl4 • Lewis Dot: • Stereochemical Formula

  12. AX3E - Trigonal pyramidal • Example: NH3 • Lewis Dot: • Stereochemical Formula

  13. AX2E2 - Angular • Example: H2O • Lewis Dot: • Stereochemical Formula

  14. .. • AXE3 - Linear • Example: HCl • Lewis Dot: H:Cl: • Stereochemical Formula: H---Cl ..

  15. Pages 91 –98 Practice Problems 1-8

  16. Dipole Theory • Polar Molecule: a molecule in which the negative charge is not evenly distributed around the molecule • Non-polar molecule: a molecule with symmetrical distribution of the negative charge.

  17. Dipole Theory

  18. Dipole Theory

  19. Theoretical Prediction of Molecular Polarity • To use molecular shape and bond polarity to predict the polarity of a molecule, complete these steps: • Step 1: Draw a Lewis formula for the molecule. • Step 2: Use the number of electron pairs and VSEPR rules to determine the shape around each central atom.

  20. Theoretical Prediction of Molecular Polarity • Step 3: Use electronegativities to determine the polarity of each bond. • Step 4: Add the bond dipole vectors to determine whether the final result is zero (nonpolar molecule) or nonzero (polar molecule). • Sample 3.5 page 102 shows this well.

  21. Pages 98-104 Read all. Take notes on Bolded terms. Copy Table 8 page 99 into notes Practice Problems 9-16 Section Questions 1-10

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