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Molecular Geometry and Bonding Theories

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The properties of a molecule depend on its shape and and the nature of its bonds. In this unit, we will discuss three 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 (3) a model of chemical bonding that deals with the electronic structure of molecules -- molecular orbital (MO) theory

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CO 2 bond angles: the angles made by the lines joining the nuclei of a molecule’s atoms CH 4 CH 2 O carbon dioxidemethaneformaldehyde 180 o 120 o o

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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. N–HH– H.. N H H H 4 e – domains NH 3 Domains arrange themselves so as to minimize their repulsions.

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The electron-domain geometry is one 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 N H H H..

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Total # of Domains Electron-Domain Geometry Possible Molecular Geometries linear linear (CO 2 ) trigonal planar trigonal planar (BF 3 ), bent (NO 2 ) tetrahedral tetrahedral (CH 4 ), trigonal pyramidal (NH 3 ), bent (H 2 O) trigonal bipyramidal trig. bipyramidal (PCl 5 ), linear (XeF 2 ) seesaw (SF 4 ), T-shaped (ClF 3 ) octahedral octahedral (SF 6 ), sq. pyr. (BrF 5 ), square planar (XeF 4 ) “atoms – axial”

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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. SnCl 3 – 26 e – [ ] Sn–ClCl–.. Cl –.. EDG: tetrahedral MG: trig. pyramidal O3O3 18 e – EDG: trig. planar MG: bent.. O –O O=.. SeCl 2 20 e – EDG: tetrahedral MG: bent Cl–Se–Cl.. O O– O=..

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CO 3 2– 24 e – [ ] C= OO–.. O 2– EDG: trig. planar MG: trig. planar SF 4 34 e – EDG: trig. bipyr. MG: seesaw.. S –F F–.. F F IF 5 42 e – I.. F –F.. –F.. F–.. F EDG: octahedral MG: sq. pyramidal ClF 3 28 e – Cl –F F–.. F EDG: trig. bipyr. MG: T-shaped (res.)

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