VSEPR & Geometry

1. VSEPR & Geometry • Lewis structures show the number and type of bonds between atoms in a molecule or polyatomic ion. • Lewis structures are not intended to show the 3-dimensional structure (i.e. shape or geometry) of the molecule. • The shape of a molecule is determined by the bond angles and bond lengths between the atoms.

2. H H O VSEPR & Geometry • Bond length:the distance between two atoms held together by a chemical bond • Bond length is affected by the number of bonds between the two atoms. • Single bonds are longest. • Triple bonds are shortest. • Bond angle: the angle made by the imaginary lies joining the nuclei of the atoms in a molecule 104.5o

3. VSEPR & Geometry • Many of the molecules we have discussed have a central atom surrounded by two or more “outer” atoms: ABn where A = central atom B = outer atom n = # of “B” atoms Examples: CO2, H2O, BF3, NH3, CCl4, CHCl3

4. H O H VSEPR & Geometry • The shape of ABn molecules depends, in part, on the value of n. • AB2 molecules can either be linear or bent. CO2 linear O O C H2O bent

5. H H B H VSEPR & Geometry • AB3 molecules can be trigonal planar, trigonal pyramidal, or T-shaped. • Trigonal planar: • Atom “A” in the center of an equilateral triangle with “B” atoms at each corner. All atoms in the same plane. BH3

6. H H N H VSEPR & Geometry • Trigonal pyramidal: • The three “B” atoms are arranged at the corners of an equilateral triangle • Central atom “A” is located in the center but above the plane of the triangle. NH3

7. F F Cl F VSEPR & Geometry • T-shaped: ClF3

8. VSEPR & Geometry • How do you decide which AB3 molecules are trigonal planar, which are trigonal pyramidal, and which are T-shaped? • If the central atom “A” is a main group element, the valence shell electron-pair repulsion model (VSEPR) can be used to predict the shape of a molecule (or polyatomic ion).

9. VSEPR & Geometry • VSEPR counts the number of electron domains around the central atom and uses this number to predict the shape. • Electron domain: • A region around the central atom where electrons are likely to be found • Two types of electron domains: • Bonding electron domains • Nonbonding electron domains

10. VSEPR & Geometry • Bonding electron domains • Also called bonding electrons • Electrons that are shared between two atoms CCl4 has 4 bonding pairs of electrons

11. VSEPR & Geometry • Nonbonding electron domains • Also referred to as nonbonding pairs or lone pairs of electrons: • Electrons that are found principally on one atom • Unshared electrons

12. VSEPR & Geometry Example: Count the number of electron domains around the central atom in each of the following Lewis structures.

13. VSEPR & Geometry • Since electron domains are regions of high electron density, they tend to repel each other. • According to VSEPR, the best arrangement of a specified number of electron domains is the one that minimizes repulsions between them by placing them as far away from each other as possible.

14. Electron Domain Geometry • The electron domain geometry is found by counting the number of electrons domains and considering the arrangement that minimizes repulsions. • Electron domain geometry: the arrangement of the electron domains around the central atom • Linear • Trigonal planar • Tetrahedral • Trigonal bipyramidal • Octahedral

15. Electron Domain Geometry 2electron domains Linear electron domain geometry 3electron domains Trigonal planar e- domain geometry

16. Electron Domain Geometry 4electron domains Tetrahedral electron domain geometry Trigonal bipyramidal e- domain geometry 5electron domains

17. Electron Domain Geometry 6electron domains Octahedral electron domain geometry

18. Electron Domain Geometry • Drawing electron domain geometries in 3-d: Trigonal planar Tetrahedral

19. Electron Domain Geometry • Drawing electron domain geometries in 3-d: Trigonal bipyramidal octahedral

20. Electron Domain Geometry • To determine the name of the electron domain geometry: • Draw the Lewis structure • Count the number of electron domains around the central atom • Double bonds and triple bonds count as 1 electron domain • Assign the name of the electron domain geometry. • You should also be able to draw a 3-dimensional structure for a given substance.

21. Electron Domain Geometry Example: Determine the name of the electron domain geometry for each of the following and draw an appropriate 3-dimensional structure for it. NO2- ClF3

22. Electron Domain Geometry Example: Determine the name of the electron domain geometry for each of the following and draw an appropriate 3-dimensional structure for it. CH4 XeF4

23. Electron Domain Geometry Example: Use the Lewis structure shown below for acetone, the major component of nail polish remover, to identify the electron domain geometry around each carbon atom.

24. Molecular Geometry • Water has tetrahedral electron domain geometry: • The shape of the molecule itself, however, is not tetrahedral. • Water has a bentmolecular geometry.

25. Molecular Geometry • Molecular geometry: • The arrangement in space of the atoms in a molecule or polyatomic ion • Molecular geometry is a consequence of the electron domain geometry. • Lone pairs of electrons take up space around the central atom. • The atoms in the molecules occupy positions around the central atom that minimizes repulsion between all of the electron domains.

26. Molecular Geometry • Each type of electron domain geometry gives rise to certain specific types of molecular geometries. • The electron domain geometry and molecular geometry are the same only if all of the electron domains are bonding domains. Tetrahedral e.d. and molecular geometry

27. Molecular Geometry • Tables 9.2 and 9.3 list all possible molecular geometries for each of the five electron domain geometries. • You must be able to determine the name of the e.d. geometry and the molecular geometry. • You must be able to draw each geometry in 3-dimensions.

30. Molecular Geometry • To determine the name of the molecular geometry: • Draw the Lewis structure • Count the total # of electron domains • Identify the electron domain geometry • Determine the molecular geometry by considering the arrangement of the bonded atoms.

31. Molecular Geometry Example: Determine the name of the molecular geometry for each of the following and draw an appropriate 3-dimensional structure for it. NO2- ClF3

32. Molecular Geometry Example: Determine the name of the molecular geometry for each of the following and draw an appropriate 3-dimensional structure for it. CH4 XeF4

33. Molecular Geometry Example: Determine the name of the molecular geometry for each of the following and draw an appropriate 3-dimensional structure for it. I3- SF4