Lecture B2
VSEPR Theory
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Understanding Molecular Geometries & Electron Pair Repulsion in VSEPR & Covalent Bonding, Lecture notes of Chemistry
An in-depth exploration of VSEPR theory and covalent bonding, focusing on the prediction of molecular geometries based on electron pair repulsion. Valence Bond theory, Molecular Orbital theory, and the history of Lewis' failed attempts at a geometrical model for atoms and chemical bonding. It also includes examples of various molecular geometries, such as linear, trigonal, tetrahedral, square planar, and octahedral, and their corresponding electron pair arrangements.
What you will learn
- What are the different types of molecular geometries and their corresponding electron pair arrangements?
- How does electron pair repulsion affect molecular geometry?
- How does the number of bonds and lone pairs around an atom influence its molecular geometry?
- What is the difference between Valence Bond theory and Molecular Orbital theory?
- What is VSEPR theory and how is it used to predict molecular geometries?
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2021/2022
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Lecture B VSEPR Theory
Covalent Bond Theories
- VSEPR (valence shell electron pair repulsion model). A set of empirical rules for predicting a molecular geometry using, as input, a correct Lewis Dot representation.
- Valence Bond theory. A more advanced description of orbitals in molecules. We emphasize just one aspect of this theory: Hybrid atomic orbitals. Works especially well for organic molecules, which is the reason we don’t scrap it entirely for MO theory.
- Molecular Orbital theory. The most modern and powerful theory of bonding. Based upon QM.
G. N. Lewis tried to develop a geometrical model for atoms and chemical bonding -- but failed. G. N. Lewis 1875-
Gillespie and Nyholm devised a simple scheme for geometry based on the Lewis dot structure (VSEPR). Valence shell electron pair repulsion (VSEPR) theory is a model in chemistry used to predict the shape of individual molecules based upon the extent of electron-pair electrostatic repulsion. It is also named Gillespie-Nyholm* theory after its two main developers. The acronym "VSEPR" is pronounced "vesper" for ease of pronunciation. Ronald J. Gillespie 1924 - *Ronald J. Gillespie and Ronald S. Nyholm University College, London, 1957.
AX 2
CO 2
O-C-O angle: 180° Linear Geometry Gillespie and Nyholm looked at the structures of molecules of the form AXn: 16 electrons
O=C=O
AX 2
X-A-X angle: 180° Linear Geometry
AX 3
BF 3
F-B-F angle: 120° Trigonal Planar Geometry
F B F
F
24 electrons
AX 4
X-A-X angle: 90° Square Planar Geometry?
AX 4
CH 4
H-C-H angle: 109.47° Tetrahedral Geometry from SketchUp
AX 4
H-C-H angle: 109.47° Tetrahedral Geometry from SketchUp Green Triangle: 1 + 1 = 2 Hypotenuse = √ 2 Gray Triangle: 2 + 1 = 3 Hypotenuse = √ 3 Grey Angle = cos
- (√2/√3) Blue Triangle: Blue Angle = 180 - 2 x GA
AX 4
CH 4
H-C-H angle: 109.47° Tetrahedral Geometry
C
H
8 electrons
H H
H
AX 5
X-A-X angle: 72° Pentagonal Planar Geometry?
AX 6
X-A-X angle: 60° Hexagonal Geometry?
AX 6
SF 6
F-S-F angle: 90° Octahedral Geometry 48 electrons