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Valence Shell Electron Pair Repulsion (VSEPR) theory

Valence Shell Electron Pair Repulsion (VSEPR) theory. Mrs. Kay. Assumptions for VSEPR. Atoms in a molecule are bound together by electron pairs. These are called bonding pairs . More than one set of bonding pairs of electrons may bind any two atoms together (multiple bonding).

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Valence Shell Electron Pair Repulsion (VSEPR) theory

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  1. Valence Shell Electron Pair Repulsion (VSEPR) theory Mrs. Kay

  2. Assumptions for VSEPR • Atoms in a molecule are bound together by electron pairs. These are called bonding pairs. More than one set of bonding pairs of electrons may bind any two atoms together (multiple bonding). 1 pair = single bond 2 pair = double bond 3 pair = triple bond

  3. Some atoms in a molecule may also possess pairs of electrons not involved in bonding. These are called lone pairs or non-bonded pairs.

  4. The bonding pairs and lone pairs around any particular atom in a molecule adopt positions in which their mutual interactions are minimized. • Electron pairs are negatively charged and will get as far apart from each other as possible.

  5. Bonding angles… • Lone pairs occupy more space than bonding electron pairs. • Double bonds occupy more space than single bonds. • LP-LP > LP-BP > BP-BP • Lone pairs spread out more than bonding pairs, therefore lone pairs have less repulsions than bonding pairs

  6. Where to start? • Draw the Lewis structure based upon the atom of interest. Ex: CO2 • Determine the central atom (pick the least electronegative atom) Ex: C

  7. Determine the number bonding pairs Ex: 2 • Determine the number of lone pairs or non-bonding pairs. Ex: 0 sets = bonding pairs (single, double, or triple)

  8. 5. Using the chart figure out the shape.

  9. Put to memory!! • The predicted geometry of the molecule is based on the number of

  10. Make your table: • As a group we will organize a table that will help you decide molecular shapes.

  11. Use VSEPR to predict the following shape: SO2 SO3 [SO4 ]-2 AsCl3 SI2 CH3F CH2F2 NH4+ NO2- NO2+ H3O+ Practice:

  12. Advanced chemistry ONLY • You are responsible for 2 extra shapes • Triagonal bipyramidal; 5 electron pairs • Octahedral; 6 electron pairs

  13. Trigonal Bipyramidal • Molecules shaped by five electron pairs • Example:PCl5 phosphorous pentachloride

  14. Elements in row 3,4,... have access to d orbitals (unlike row 2 elements that have only s and p orbitals) and can therefore have more than 8 electrons around them in certain compounds.  • Recall that P has a normal valence of 3 with one lone pair.  • If that lone pair is divided into two extra unpaired electrons to be used in creating bonds, we then have a hypervalence of 5.  

  15. Octahedral Six bonding sets • Example:hexafluorophosphide [PF6]-

  16. On the computer • Exercise to practice on calculator: • http://www.shef.ac.uk/chemistry/vsepr/jmol/exercises.html

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