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Learning Objectives
By the end of this section, you will be able to:
Predict the structures of small molecules using valence shell electron pair repulsion (VSEPR) theory (4.6)
Explain the concepts of polar covalent bonds and molecular polarity (4.6)
Assess the polarity of a molecule based on its bonding and structure (4.6)
Lewis Diagrams and VSEPR Models (Bozeman): https://youtu.be/xNYiB_2u8J4
Valence-Shell Electron-Pair Repulsion theory (VSEPR)
Valence-Shell Electron-Pair Repulsion theory (VSEPR)
Valence shell electron pair repulsion theory, or VSEPR theory is a model used in chemistry to predict the geometry of individual molecules from the number of electron pairs surrounding their central atoms. The premise of VSEPR is that the valence electron pairs surrounding an atom tend to repel each other and will, therefore, adopt an arrangement that minimizes this repulsion. This, in turn, decreases the molecule's energy and increases its stability.
Linear
In chemistry, the linear molecular geometry describes the geometry around a central atom bonded to two other atoms (or ligands) placed at a bond-angle of 180°, (CO & CO2)
Trigonal Planar
In chemistry, trigonal planar is a molecular geometry model with one atom at the center and three atoms at the corners of an equilateral triangle, called peripheral atoms, all in one plane. In an ideal trigonal planar species, all three ligands are identical and all bond angles are 120°, (BF3)
Tetrahedral
In a tetrahedral molecular geometry, a central atom is located at the center with four substituents that are located at the corners of a tetrahedron. The bond angles are ≈ 109.5° when all four substituents are the same, as in methane (CH4).
Observe how the electron clouds interact and electrostatic repulsion drives the shape of the molecule.
Determining electron geometry: The AXE method
Determining electron geometry: The AXE method
The "AXE method" of electron counting is commonly used when applying the VSEPR theory.
The “A” represents the central atom and always has an implied subscript of one.
The “X” represents the number of ligands (atoms bonded to A).
The “E” represents the number of lone/unbonded electron pairs surrounding the central atom.
The sum of X and E is known as the steric number. Single, double and triple bonds all count as one bonding pair “X” because they act as one negative domain.
Molecule Shapes (PhET): https://phet.colorado.edu/sims/html/molecule-shapes/latest/molecule-shapes_en.html
On the model tab - explain how the electron domains (bonded & lone) drive the general shape of the molecule? (# 3 & 5)
Both bonded and nonbonded electron pairs (domains) repel other electron domains creating a 3D orientation.
On the real molecule tab - describe how lone pairs (compared to bonded pairs) further affect bond angles in comparison to the generic models. (# 4, 6 & 7)
the lone pairs seem to repel the bonded pairs a little more then the other bonded pairs.
Discuss how electron and molecular geometry is due to repulsions between electron groups.
Differentiate between electron domain geometry and molecular geometry.
Describe how lone pairs affect (alter) the bond angles in real molecules compared to how bonded pairs affect the angles.
Compare bond angle predictions from the VSEPR-based model to real molecules angles.
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