6.3 Hybridization and molecular geometry

Hybridization blends atomic orbitals, shaping molecules and influencing bonding. It's key to understanding how atoms connect and form specific structures. This concept ties into molecular orbital theory by explaining electron behavior in chemical bonds.

VSEPR theory predicts molecular shapes based on electron pair repulsion. It's crucial for grasping 3D molecular structures and their properties. This links to electronic structure by showing how electron arrangement affects molecular geometry.

Hybridization and Bonding

Hybrid Orbitals and Bond Types

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• Hybridization involves the mixing of atomic orbitals to form new hybrid orbitals with specific shapes and orientations
• sp hybridization occurs when one s and one p orbital combine to form two sp hybrid orbitals arranged in a linear geometry (180° bond angle)
• sp2 hybridization occurs when one s and two p orbitals combine to form three sp2 hybrid orbitals arranged in a trigonal planar geometry (120° bond angles)
• sp3 hybridization occurs when one s and three p orbitals combine to form four sp3 hybrid orbitals arranged in a tetrahedral geometry (109.5° bond angles)
• Sigma (σ) bonds are formed by the direct overlap of atomic orbitals along the internuclear axis resulting in a single covalent bond (C-C single bond)
• Pi (π) bonds are formed by the sideways overlap of unhybridized p orbitals resulting in a weaker secondary covalent bond (C=C double bond)

Valence Bond Theory

• Valence bond theory describes the formation of covalent bonds through the overlap of atomic orbitals
• Hybrid orbitals are used to explain the observed molecular geometries and bond angles in molecules
• The number of hybrid orbitals formed is equal to the number of atomic orbitals that participate in the hybridization process
• Hybrid orbitals are oriented in space to minimize electron repulsion and maximize bond stability
• The type of hybridization (sp, sp2, or sp3) determines the shape and bond angles of the molecule (linear, trigonal planar, or tetrahedral)
• Valence bond theory provides a qualitative understanding of chemical bonding but does not account for the delocalization of electrons in molecules

Molecular Geometry

VSEPR Theory and Molecular Shapes

• VSEPR (Valence Shell Electron Pair Repulsion) theory predicts the geometry of molecules based on the repulsion between electron pairs
• Molecular geometry refers to the three-dimensional arrangement of atoms in a molecule determined by the number and type of electron pairs (bonding and nonbonding) around the central atom
• Electron pairs (bonding and nonbonding) repel each other and arrange themselves to minimize repulsion leading to specific molecular geometries (linear, trigonal planar, tetrahedral, trigonal bipyramidal, or octahedral)
• Lone pairs of electrons occupy more space than bonding pairs resulting in slightly distorted geometries (bent, trigonal pyramidal, or seesaw) compared to the ideal geometries predicted by VSEPR theory

Bond Angles and Molecular Polarity

• Bond angles are the angles formed between the imaginary lines connecting the nuclei of the bonded atoms in a molecule
• The ideal bond angles for common geometries are 180° (linear), 120° (trigonal planar), 109.5° (tetrahedral), 90° and 120° (trigonal bipyramidal), and 90° (octahedral)
• The presence of lone pairs causes a slight decrease in bond angles due to their greater repulsive effect compared to bonding pairs (H2O bond angle is 104.5° instead of the ideal 109.5° for a tetrahedral arrangement)
• Molecular polarity depends on the geometry of the molecule and the polarity of individual bonds (polar molecules have an uneven distribution of charge, while nonpolar molecules have a balanced distribution of charge)
• Molecules with symmetric geometries and no lone pairs are typically nonpolar (CO2 is linear and nonpolar), while molecules with asymmetric geometries or lone pairs are typically polar (NH3 is trigonal pyramidal and polar)