Transcript Slide 1
ELECTRON & MOLECULAR GEOMETRY • VSEPR • Electron geometries • Molecular geometries 1 VSEPR Valence shell electron-pair repulsion • Theory is used to predict molecular geometry by examining number of bonds & unshared electron pairs. • Most stable arrangement is one where valence electrons around central atom are as far away from each other as possible. – Minimizes repulsions 2 ICl4 Why is the top structure incorrect? Bottom structure places 2 electron pairs 180° away from each other whereas the top structure places them at 90° away from each other 3 • Each bond on central atom counts as 1 group of electrons. Single / double / triple : all count as 1 group • Each lone pair on central atom counts as 1 group of electrons. Even though lone pairs are not attached to other atoms, they “occupy space” around central atom H O C O (0) (0) (0) H N H 4 Electron group geometry 2 groups = linear 3 groups = trigonal planar 4 groups= tetrahedral 5 groups = trigonal bipyramidal 6 groups = octahedral 5 H H N H - 4 electron groups - tetrahedral electron-group geometry - molecular shape (molecular geometry) is not tetrahedral but rather trigonal-pyramidal 6 • Electron-group geometry deals with the distribution of the electron groups • Molecular geometry deals with the molecular shape of the molecule VSEPR notation AXE A = central atom X = ligand(s) attached to central atom E = lone pair electrons on central atom *Know bond angles too 8 Electron groups Electron geometry Lone VSEPR pairs notation Molecular geometry Bond angles E.g. 2 Linear 0 AX2 linear 180 BeCl2 3 Trigonal planar 0 AX3 trigonal planar 120 BF3 1 AX2E angular 120 SO2 0 AX4 tetrahedral 109.5 CH4 1 AX3E trigonal pyramidal 109.5 NH3 2 AX2E2 angular 109.5 H2O 4 Tetrahedral Electron groups Electron geometry 5 Trigonal bipyramidal 6 Octahedral Lone VSEPR pairs notation Molecular geometry Bond angles E.g. 0 AX5 trigonal bipyramidal 120, 90 PCl5 1 AX4E seesaw 120, 90 SF4 2 AX3E2 T-shaped 90 ClF3 3 AX2E3 linear 180 XeF2 0 AX6 octahedral 90 SF6 1 AX5E square pyramidal 90 BrF5 2 AX4E2 Square planar 90 XeF4 AX2 AX4 AX3 AX3E AX2E AX2E2 AX5 AX2E3 AX4E AX3E2 AX6 AX5E AX4E2 Molecules With More Than One Central Atom CH3NCO H H C N C O H H H C N C O H AX4 AX2E AX2 14 linear o 180 C N tetrahedral H o 120 C H O trigonal-planar H o 109 15 Predicting a VSEPR Structure 1. Draw Lewis structure. 2. Count number of regions of high electron density (unshared pairs and bonds) around central atom. 3. Identify electron-pair geometry. 4. If more than one arrangement is possible, choose one that minimizes unshared e- pair repulsions. 5. Identify molecular geometry. 16 PRACTICE EXAMPLE 17