The VSEPR Theory Section 4.3.

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Presentation transcript:

The VSEPR Theory Section 4.3

VSEPR VSEPR (Valence Shell Electron Pair Repulsion) valence electrons stay as far apart as possible to minimize repulsion Tells us the geometry of the molecule When looking at a molecule we look specifically at the central atom (the one that has the most bonding electrons) to determine the geometry

Rules of VSEPR Only the valence shell electrons of the central atom(s) are important for molecular shape Valence shell electrons are paired or will be paired in a molecule or polyatomic ion Bonded pairs or electrons and lone pairs of electrons are treated approximately equal Valence shell electron pairs repel each other electrostatically (“like repels like”) The molecular shape is determined by the positions of the electron pairs when they are a maximum distance apart.

Representing 3-D Structures The solid wedge represents atoms/pairs of electrons coming TOWARDS the viewer The dashed wedge represents atoms/pairs of electrons going AWAY from the viewer

Using VSEPR First draw Lewis structures of the molecule, including the electron pairs around the central atom Count the total number of bonding pairs and lone pairs around the central atom Refer to table 1 on pg. 245 or Appendix C3 to predict the shape

These are the major types. Others shown in Appendix C3 include: Linear AX2 AX3 Trigonal planar Tetrahedral AX4 Trigonal pyramidal AX3E Bent AX2E2 These are the major types. Others shown in Appendix C3 include: AX5 – Trigonal bypyramidal AX6 – Octahedral AX5E – Square pyramidal AX4E2 – Square planar A represents the central atom X represents the bonded pairs E represents lone pairs of electrons

Examples What are the shapes of the following molecules: nitrogen trihydride dihydrogen sulfide methane boron trichloride

Some Things you should know… Secrets we’ve been keeping: Sulfur can have several different valence configurations: It is actually found in nature as S8 Can have 2, 4, 6 unpaired electrons Phosphorous Can have 3, 4, 5 unpaired electrons Basically the larger the atom the more electrons it can have. And some noble gases form compounds! When in doubt use the method of adding up valence electrons from yesterday to determine the # of l.p. of e- on the central atom.

phosphorous pentafluoride bromine pentachloride xenon tetrafluoride sulfur hexafluoride

Assign Geometries for: Iodine dichloride ion (ICl2-) Chlorine trifluoride (ClF3) Sulfur tetrafluoride (SF4) When you are done complete the worksheet. (we are working on this tomorrow too!)