Molecular Geometry and Bonding Theories

The properties of a molecule depend on its shape and and the nature of its bonds. In this unit, we will discuss two models. (1) a model for the geometry of molecules -- valence-shell electron-pair repulsion (VSEPR) theory (2) a model about WHY molecules form bonds and WHY they have the shape they do -- valence-bond theory

bond angles: the angles made by the lines joining the nuclei of a molecule’s atoms carbon dioxide methane formaldehyde CO2 CH4 CH2O 180o 109.5o 120o

.. .. VSEPR electron domain: a region in which at least two electrons are found -- they repel each other because… they are all (–) bonding domain: 2-to-6 e– that are shared by two atoms; they form a… covalent bond nonbonding domain: 2 e– that are located on a single atom; also called a… lone pair For ammonia, there are three bonding domains and one nonbonding domain. NH3 N –H H– H .. 4 e– domains N H .. Domains arrange themselves so as to minimize their repulsions.

.. .. N The electron-domain geometry is one H of five basic arrangements of domains. -- it depends only on the total # of e– domains, NOT the kind of each domain The molecular geometry describes the orientation of the atoms in space. .. -- it depends on how many of each kind of e– domain

Electron-Domain Geometry Possible Molecular Geometries Total # of Domains Electron-Domain Geometry Possible Molecular Geometries 2 3 4 5 6 linear linear (CO2) trigonal planar trigonal planar (BF3), bent (NO2) tetrahedral tetrahedral (CH4), trigonal pyramidal (NH3), bent (H2O) “atoms – axial” trig. bipyramidal (PCl5), linear (XeF2) seesaw (SF4), T-shaped (ClF3) trigonal bipyramidal octahedral octahedral (SF6), sq. pyr. (BrF5), square planar (XeF4)

[ ] .. .. .. .. To find the electron-domain geometry (EDG) and/or molecular geometry (MG), draw the Lewis structure. Multiple bonds count as a single domain. Predict the EDG and MG of each of the following. [ ] Sn –Cl Cl– .. Cl – SnCl3– EDG: tetrahedral MG: trig. pyramidal 26 e– .. O –O O= .. O O– O= O3 EDG: trig. planar 18 e– MG: bent Cl–Se–Cl .. SeCl2 EDG: tetrahedral 20 e– MG: bent

[ ] .. .. .. .. .. .. .. C= O O– 2– CO32– EDG: trig. planar (res.) [ ] C= O O– .. 2– CO32– EDG: trig. planar (res.) MG: trig. planar 24 e– .. S –F F– F SF4 EDG: trig. bipyr. .. MG: seesaw 34 e– I .. F –F F– IF5 EDG: octahedral .. MG: sq. pyramidal 42 e– Cl –F F– .. F ClF3 EDG: trig. bipyr. .. MG: T-shaped 28 e–