Lecture B2 - VSEPR Theory and Covalent Bond Theories - Hybrid Orbitals

Oct 29, 2026

Lecture B2 explains major covalent bonding theories with emphasis on molecular geometry and electron-pair interactions. It introduces VSEPR, based on a correct Lewis dot representation, for predicting molecular shapes from electrostatic repulsion among electron pairs. It then outlines valence bond theory, focusing on hybrid atomic orbitals and its usefulness for organic molecules, and contrasts it with molecular orbital theory grounded in quantum mechanics. The lecture also references the Lewis and Gillespie–Nyholm contributions and describes key AXn geometry cases such as linear, trigonal planar, and trigonal planar B F3.

LanguageEnglish

CategoryTechnology

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Pages72

Reading Time3 h 1 min

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VSEPR and Covalent Bond Theories

VSEPR model and Lewis dot representation

Valence bond theory and hybrid orbitals

Molecular orbital theory based on QM

Geometry for AXn molecules

Linear and trigonal geometries

Lecture B2 VSEPR Theory Covalent Bond Theories 1. 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. 2. 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. 3. Molecular Orbital theory. The most modern and powerful theory of bonding. Based upon QM. Covalent Bond Theories 1. 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. 2. 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. 3. 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 1875-1946 | | | 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 and Ronald S. Nyholm Ronald J. Gillespie University College, London, 1957. 1924- | | | Living in 3 Dimensions | | | --- | --- | | | What role does geometry play in chemical structure? | | Gillespie and Nyholm looked at the structures of molecules of the formAXn: | AX 2 | | --- | --- | |
CO 2
16 electrons
O =C= O
O-C-O angle: 180° Linear Geometry
| | | | AX 2 | | --- | --- | | X-A-X angle: 180° Linear Geometry | | | | AX 3 | | --- | --- | | X-A-X angle: 120° Trigonal Geometry | | |
BF 3
F-B-F angle: 120° Trigonal | AX 3
24 electrons

F

F ~~ ~~ B ~~ ~~ F
Planar Geometry
| | --- | --- |

FAQs

What does VSEPR theory use to predict molecular geometry?

VSEPR uses a correct Lewis dot representation as input, then applies electron-pair electrostatic repulsion to predict molecular shape.

How does valence bond theory relate to hybrid atomic orbitals?

Valence bond theory describes orbitals in molecules and highlights hybrid atomic orbitals as a key aspect, which works especially well for organic molecules.

What is the main basis of molecular orbital theory?

Molecular orbital theory is the most modern and powerful bonding theory, grounded on quantum mechanics (QM).

How are geometry types determined for AXn structures in the lecture?

The lecture explains geometry by using the X-A-X bond angles for cases such as AX2 (180° linear) and AX3 (120° trigonal planar geometry), including the example of BF3.

Oct 29, 2026

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