Questions and Answers
What is the Valence-Bond Theory?
A model of chemical bonding in which an electron pair bond is formed between two atoms by the overlap of orbitals in the two atoms.
What is an example of orbital overlap in H2?
In the coming together of 2 H atoms to form H2, as the orbitals overlap, electron density is concentrated between the nuclei.
What is an example of orbital overlap in HCl?
The Cl has a configuration of [Ne]3s23p5, and this 3p electron pairs with the single 1s electron from H to form a covalent bond.
What is the relationship between potential energy and orbital overlap?
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What is developing the concept of hybrid orbitals?
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What are hybrid orbitals?
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What is hybridization?
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What is the relationship between the number of electron domains and the number of hybrid orbitals?
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What is electron promotion?
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What is the solution to the dilemma of equivalency: 'sp' hybridization?
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What is the diagram representation of 'sp' hybrids in a molecule?
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What are the 'sp2' and 'sp3' hybrid orbitals?
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What is an example of 'sp2' orbitals in BF3?
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What is an example of 'sp3' orbital in CH4?
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What is hybrid bonding in molecules with nonbonding pairs?
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Why can't expanded-octet molecules cannot be represented by the valence-theory orbital model?
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What is the hybrid orbital summary?
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What is the step-by-step process of identifying hybrid orbitals?
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What are sigma (σ) bonds?
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What are pi bonds?
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Why are pi bonds weaker than sigma bonds?
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What is molecular architecture?
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What are Lewis structures vs. molecular shapes?
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What are bond angles?
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What is the CCl4 molecular shape?
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What are conventional ABn molecules?
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What are the possible shapes of AB2 and AB3 molecules?
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What are the five shapes that maximize separation of outer atoms?
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What is the Valence-Shell Electron-Pair Repulsion (VSEPR) model?
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What is a bonding pair?
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What is a nonbonding pair/lone pair?
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What determines the shape of a molecule?
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What is an electron domain?
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What is an example involving determining the number of electron domains?
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What is the Valence Shell Electron Pair Repulsion Theory (VSEPR)?
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What is electron-domain geometry?
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What are the electron-domain geometries as a function of the number of electron domains?
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What is molecular geometry?
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How do you predict the shape of a molecule (molecular geometry)?
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What is the step-by-step process of predicting molecular geometry?
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What is an example of determining the molecular geometry of an NH3 molecule?
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What is an example of predicting molecular geometry of CO2?
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What are the electron domain and molecular geometries for two, three, and four electron domains around a central atom?
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What is the linear electron domain?
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What is the trigonal planar electron domain?
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What is the tetrahedral electron domain?
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What is an example involving predicting molecular geometries?
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What is the effect of nonbonding electrons and multiple bonds on bond angles?
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What are the physical differences between nonbonding and bonding electron pairs?
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What is the effect of multiple bonds on bond angles?
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What is an example of the effect of multiple bond angles on CCl2O?
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What are the electron domain geometries of molecules with expanded valence shells?
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What are the properties of five electron-domains (trigonal bipyramidal)?
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What are axial positions?
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What are equatorial positions?
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What are the electron-domain and molecular geometries for five and six electron domains around a central atom?
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What is the trigonal bipyramid electron domain?
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What is the octahedral electron domain?
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What are all possible molecular geometries as a function of ABn?
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What are six electron domains (octahedral)?
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What is an example involving determining the molecular geometries of molecules with expanded valence shells?
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What are the shapes of larger molecules that deviate from the ABn convention?
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What is an example involving determining the shape of acetic acid?
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What is an example involving predicting bond angles of larger molecules?
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What is a bond dipole?
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What is the overall dipole moment of a molecule?
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What is an example of determining polarity of a CO2 molecule?
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What is an example of determining polarity of an H2O molecule?
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What are the six symmetrical shapes for which ABn molecules must be nonpolar?
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What are some examples of polar and nonpolar molecules containing polar bonds?
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What is an example involving determining the polarity of molecules?
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What is the development of the valence-bond theory?
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Study Notes
Molecular Architecture
- Molecular architecture refers to the shape and size of molecules, critical for drug effectiveness.
- The relationship between two-dimensional Lewis structures and three-dimensional shapes is essential.
Lewis Structures vs. Molecular Shapes
- Lewis structures depict electron arrangements, while molecular shapes represent the spatial arrangement of atoms.
Bond Angles
- Bond angles are formed by lines connecting atom nuclei in a molecule.
- The shape and size of a molecule depend on bond angles and lengths.
CCl4 Molecular Shape
- Carbon tetrachloride (CCl4) has a tetrahedral shape with bond angles of 109° and bond lengths of 1.78 Å.
Conventional ABn Molecules
- ABn molecules consist of a central atom (A) bonded to "n" identical atoms (B).
- The shape of ABn molecules varies with "n"; examples include CO2 (AB2) and NH3 (AB3).
Possible Shapes of AB2 and AB3 Molecules
- AB2 can be linear (180°) or bent.
- AB3 typically has trigonal planar (120°) or trigonal pyramidal shapes, with some being T-shaped.
Maximum Separation Shapes
- Five shapes maximize distances between outer atoms: trigonal bipyramidal and octahedral.
Valence-Shell Electron-Pair Repulsion (VSEPR) Model
- This model predicts geometric arrangements based on electron pair repulsions around a central atom.
- It helps explain molecular shapes, especially for representative elements.
Bonding Pair
- A bonding pair of electrons is shared between two atoms, defining a region of electron density.
Nonbonding Pair/Lone Pair
- A nonbonding pair of electrons is located on one atom; affects molecular shape but not the structure directly.
Determinants of Molecular Shape
- Electron pairs repel each other, allowing predictions on molecule shape based on their arrangement.
Electron Domain
- An electron domain consists of bonding or nonbonding pairs around a central atom.
Predicting Molecular Geometry
- The VSEPR model aids in discerning the electron domain geometry, guiding the prediction of molecular shapes.
Molecular Geometry Definition
- Molecular geometry pertains to the arrangement of only the atoms in a molecule, excluding nonbonding pairs.
Effects of Nonbonding Electrons on Bond Angles
- The presence of nonbonding pairs alters bond angles compared to idealized geometries due to increased repulsion.
Multiple Bonds and Bond Angles
- Multiple bonds exert greater repulsive forces compared to single bonds, influencing adjacent bond angles.
Expanded Valence Shells
- Atoms in period 3 and beyond can accommodate more than four electron pairs, resulting in octahedral or trigonal bipyramidal geometries.
Electron-Domain Geometries
- Various geometries arise from two to six electron domains around a central atom, influencing molecular shapes.
Symmetrical Shapes and Molecular Polarity
- Certain symmetrical configurations lead to nonpolar molecules despite individual bond polarities.
Bond Dipole and Overall Dipole Moment
- A bond dipole is linked to unequal electron sharing, contributing to the overall dipole moment, derived from vector summation of individual bond dipoles.
Valence-Bond Theory Development
- This theory explains chemical bonding as resulting from orbital overlap, focusing on the concentration of electron pairs between atoms.
Orbital Overlap Examples
- In H2, electron density forms between nuclei due to overlapping orbitals.
- HCl exhibits orbital overlap between hydrogen and chlorine, leading to covalent bonding.
Relationship Between Potential Energy and Orbital Overlap
- Optimal overlap reduces energy due to increased attraction, but excessive proximity raises energy due to nuclear repulsion.
Concept of Hybrid Orbitals
- Hybrid orbitals reconcile bonding through orbital overlap with geometrical shapes predicted by the VSEPR model.### Hybrid Orbitals
- Hybrid orbitals are formed by mixing different atomic orbitals (e.g., sp3 hybrid from one s and three p orbitals).
- They explain the equivalent bond formations around a central atom.
Hybridization
- Mixing atomic orbitals generates equivalent hybrid orbitals.
- This mathematical process aids in understanding bond equivalency around central atoms.
Electron Domains and Hybrid Orbitals
- The number of hybrid orbitals corresponds to the number of atomic orbitals mixed.
- Specific hybridizations relate to molecular geometries:
- linear (sp),
- bent (sp2, sp3),
- trigonal planar (sp2),
- tetrahedral (sp3).
- The number of electron domains determines the type of hybridization.
sp Hybrid Orbitals
- "sp" hybridization involves mixing one s and one p orbital to create two equivalent sp hybrid orbitals.
- These orbitals point in opposite directions, facilitating linear arrangements.
Diagram Representation of sp Hybrids
- sp hybrid orbitals are depicted with one large lobe and one smaller lobe.
- The hybridization supports a linear arrangement of electron domains.
sp2 and sp3 Hybrid Orbitals
- Combining different atomic orbitals produces equivalent hybrid orbitals that point in varying directions.
- sp2 hybrid orbitals (2s + 2p) yield a trigonal planar geometry, while sp3 hybrid orbitals (1s + 3p) yield tetrahedral geometry.
Examples of sp2 Orbitals in BF3
- In BF3, three equivalent sp2 orbitals form bonds with fluorine atoms, leading to a trigonal planar structure with 120° angles.
- An unhybridized p orbital remains.
Examples of sp3 Orbital in CH4
- CH4 features four sp3 hybrid orbitals arranged tetrahedrally, facilitating distinct bonding with hydrogen atoms.
Hybrid Bonding with Nonbonding Pairs
- Hybridization also describes bonding in molecules with lone pairs, as seen in H2O, where two sp3 hybrid orbitals contain lone pairs, and the other two form bonds with hydrogen atoms.
Limitations of the Valence Theory Orbital Model
- Expanded-octet molecules require more than the standard model allows, as hybridization must consider energy levels.
- For hypervalent elements, the model fails to effectively represent bonding in compounds like SF6.
Hybrid Orbital Summary
- Knowing the electron domain geometry allows prediction of hybridization types:
- 2 electron domains = sp (linear)
- 3 electron domains = sp2 (trigonal planar)
- 4 electron domains = sp3 (tetrahedral)
Process of Identifying Hybrid Orbitals
- Draw the molecule's Lewis structure.
- Use VSEPR theory to ascertain electron domain geometry.
- Identify the corresponding hybrid orbitals based on the arrangement of electron pairs.
Valence Bond Theory
- Covalent bonds form through overlapping atomic orbitals, resulting in sigma (σ) and pi (π) bonds.
- Sigma bonds involve electron density between atomic nuclei, providing strong bond strength.
Sigma Bonds
- Characterized by concentrated electron density along the internuclear axis.
- Formed via overlaps of atomic orbitals (s-s or s-p).
Pi Bonds
- Feature electron density above and below the internuclear axis.
- A double bond consists of one σ and one π bond; a triple bond consists of one σ and two π bonds.
Strength of Bonds
- π bonds are generally weaker than σ bonds due to the less effective sideways overlap of p orbitals.
- σ bonds benefit from strong attraction between electron clouds and atomic nuclei due to their positioning.
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Explore the fundamental concepts of Molecular Architecture and Lewis Structures through these flashcards. Understand how the shape and size of molecules influence drug effectiveness and the distinction between two-dimensional representations and three-dimensional molecular shapes.