1 Class 8: Introduction to VSEPR Theory Sec 10.2 VSEPR Theory: The Five Basic Shapes Two Electron Groups: Linear Geometry Three Electron Groups: Trigonal Planar Geometry Four Electron Groups: Tetrahedral Geometry Five Electron Groups: Trigonal Bipyramidal Geometry Six Electron Groups: Octahedral Geometry Sec 10.3 VSEPR Theory: The Effect of Lone Pairs Four Electron Groups with Lone Pairs Five Electron Groups with Lone Pairs Six Electron Groups with Lone Pairs 2 Shapes of Molecules While Lewis theory does an excellent job describing the behavior of valence electrons in molecules, it does not provide information about the shapes of molecules. Remember: Class #4 Lewis theory predicts water to be linear, however, the molecule is actually bent Distinct molecular shapes arise due to the distances between bonded atoms and the angle between those bonds 104.5 o 98.5 pm 3 Shapes of Molecules Electron pairs repel each other, whether they are in chemical bonds (bond pairs) or unshared (lone pairs). Electron pairs assume orientations about an atom to minimize repulsions. Each electron group wants to get as far away as it possibly can from all other electron groups this is the basis for VSEPR theory Introduction to VSEPR Theory 1
4 VSEPR Theory Valence-Shell Electron-Pair Repulsion (VSEPR) Theory a theory used to predict probable shapes of molecules (or polyatomic ions) based on the repulsions of electron pairs found in the valence shell of the central atom of the structure. VSEPR theory focuses on electron groups. An electron group is defined as: One lone pair All of these are considered One single bond to be equivalent when One double bond One triple bond determining the shape of a One odd (unpaired) electron molecule or ion 5 VSEPR Theory 6 Shapes Electronic geometry (or shape) Is the shape that best describes the arrangement of all electron groups (bonding and lone pairs) in the valence shell of the central atom Molecular geometry (or shape) Is the actual shape of the molecule or polyatomic ion only the atoms that make up the species are considered Any lone pairs are ignored In a species where all the electron groups are bonding groups, the molecular and electronic shapes will be the same Otherwise, the two are related, but not the same. Introduction to VSEPR Theory 2
7 Terminology A = central atom X = terminal atom (or a group of atoms) bonded to the central atom E = lone pair of electrons 8 The 5 Basic Shapes The 5 basic arrangements of electron-groups around a central atom are: Two electron groups = linear Three electron groups = trigonal-planar Four electron groups = tetrahedral Five electron groups = trigonal-bipyramidal Six electron groups = octahedral 9 The 5 Basic Shapes Introduction to VSEPR Theory 3
10 Two Electron Groups Linear Two electron groups = linear = AX 2 In order for the two electron groups around the central atom to be as far apart as possible, the atom form an angle of 180 o. Example: BeCl 2 11 Three Electron Groups Trigonal Planar Three electron groups = trigonal planar = AX 3 In order for the three electron groups to be as far apart as possible, they lie in a plane with bond angles of 120 o. Example: BCl 3 12 Three Electron Groups Trigonal Planar We still call formaldehyde trigonal planar, however, because the bonds are not identical, the observed angles are slightly different from the ideal. Introduction to VSEPR Theory 4
13 Four Electron Groups -- Tetrahedral Four electron groups = tetrahedral = AX 4 In order for the four electron groups to be as far apart as possible, they must point to the corners of a tetrahedron (a 4-sided polyhedron). Example: CH 4 14 Drawing 3D on Paper Dashed wedge indicates this bond is pointing away from you (into the paper) Solid wedge indicates this bond is pointing toward you (out of the paper) 15 Five Electron Groups Trigonal Bipyramidal The positions above and below the central atom are called the axial positions (like the axis of the Earth). The positions in the same base plane as the central atom are called the equatorial positions (like the equator of the Earth). The bond angle between equatorial positions is 120. The bond angle between axial and equatorial positions is 90. Introduction to VSEPR Theory 5
16 Five Electron Groups Trigonal Bipyramidal Five electron groups = trigonal bipyramidal = AX 5 Example: PF 5 17 Six Electron Groups Octahedral Six electron groups = octahedral = AX 6 All bonds are at 90 o angles. All atoms point toward the corners of an octahedron (8-sided polyhedron) Example: SF 6 18 The Effect of Lone Pairs on Shape The closer two groups of electrons are forced, the stronger the repulsion between them Example: The repulsive force between two electron groups separated by an angle of 90 o is greater than it would be at 120 o or 180 o. Lone-pair electrons spread out more than bond-pair electrons do Lone pairs occupy more space than bond-pairs Lone pairs are held much closer to the central atom in comparison to bond pair So, the order of repulsive forces, from strongest to weakest, is: Lone-pair to Lone-pair Lone-pair to Bond-pair Bond-pair to Bond-pair Introduction to VSEPR Theory 6
19 Three Electron Groups with Lone Pairs AX 2 E Electron geometry Trigonal Planar Molecular geometry Bent Electron geometry reflects the positions of all electron groups. Molecular geometry reflects the positions of just the atoms. 20 Four Electron Groups with Lone Pairs AX 3 E Electron geometry Tetrahedral Molecular geometry Trigonal pyramidal The lone pairs repel the bonding pairs more, so bond angles between atoms are < 109.5 o. 21 Four Electron Groups with Lone Pairs AX 2 E 2 Electron geometry Tetrahedral Molecular geometry Bent The two lone pairs further repel the bonding pairs, so bond angles are approximately 104.5 o. Introduction to VSEPR Theory 7
22 Five Electron Groups with Lone Pairs AX 4 E Electron geometry Trigonal Bipyramidal Molecular geometry Seesaw The lone pairs occur in the equatorial positions in order to reduce repulsions. Example: IF 2 O 2-23 Five Electron Groups with Lone Pairs AX 3 E 2 Electron geometry Trigonal Bipyramidal Molecular geometry T-Shaped 24 Five Electron Groups with Lone Pairs AX 2 E 3 Electron geometry Trigonal Bipyramidal Molecular geometry Linear Introduction to VSEPR Theory 8
25 Six Electron Groups with Lone Pairs AX 5 E Electron geometry Octahedral Molecular geometry Square Pyramidal F (axial) -Br-F (equatorial) bond angle is less than 90 o. 26 Six Electron Groups with Lone Pairs AX 4 E 2 Electron geometry: Octahedral Molecular geometry: Square Planar Example: ICl 4-27 Molecular Geometry - Summary Introduction to VSEPR Theory 9
28 On Wednesday Sec 10.4 VSEPR Theory: Predicting Molecular Geometries Predicting the Shapes of Larger Molecules Sec 10.5 Molecular Shape and Polarity And, of course, Western Conference Finals Game 2! @ Introduction to VSEPR Theory 10