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HL4-2

VSEPR Theory 4.2.7 14.1.1. HL4-2.ppt. Molecular Geometry - VSEPR. One way to predict the shape of a molecule is the Valence Shell Electron Pair Repulsion (VSEPR) Theory. Q – Do electrons like to hang out together? A – Heck no!

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HL4-2

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  1. VSEPR Theory 4.2.7 14.1.1 HL4-2.ppt

  2. Molecular Geometry - VSEPR • One way to predict the shape of a molecule is the Valence Shell Electron Pair Repulsion (VSEPR) Theory. • Q – Do electrons like to hang out together? • A – Heck no! • The basic idea is that electron pairs in a molecule, whether being shared or not, will move as far apart as possible to minimize the repulsion between them.

  3. Molecular Geometry - VSEPR • VSEPR Steps • Write the Lewis electron-dot structure for the compound. • Determine the number of electron pair groups (Steric Number) surrounding the central atom(s). • Double and triple bonds count as one pair • Determine the geometric shape that maximizes the distance (bond angle) between the electron groups.

  4. Example – BeCl2 • How many electron pair groups? • 2 • When you have 2 pair groups the shape is linear.

  5. Example – BCl3 • How many electron pair groups? • 3 • When you have 3 pair groups the shape is trigonal planar.

  6. Example – CH4 • How many electron pair groups? • 4 • When you have 4 pair groups the shape is tetrahedral.

  7. Example – PCl5 • How many electron pair groups? • 5 • When you have 5 pair groups the shape is trigonal bipyramidal.

  8. Example – SF6 • How many electron pair groups? • 6 • When you have 6 pair groups the shape is octahedral.

  9. Example – H2O • How many electron pair groups? • 4 • When you have 4 pair groups the shape is tetrahedral. • Is that what water looks like? • Heck no!

  10. Example – H2O • When you have tetrahedral geometry but two of the sides are unshared pairs of electrons, you end up with a bent shape. • In the bent shape, the electron pairs are there in a tetrahedral form but we don’t see them.

  11. Question • What would happen to the bond angle if there was 1 lone pairs instead of two? • The bond angle would continue to increase.

  12. Example – NH3 • How many electron pair groups? • 4 • When you have 4 pair groups the shape is tetrahedral, but if one is a lone electron pair, we get a trigonal pyramidal.

  13. More on bond angles • What is a little odd about the geometries of CH4 and NH3?

  14. More on bond angles • Even though both CH4 and NH3 follow a tetrahedral pattern, they don’t have the same bond angles because a lone pair repels a little harder than a bonding pair.

  15. Homework • VSEPR Theory Wkst • Try a couple tonight, entire worksheet (both sides) is due Monday.

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