Molecular Geometry

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Molecular Geometry Bonding Theories

• Molecular Shapes
• VSEPR Model
• Molecular Polarity
• Covalent Bonding
• Hybrid Orbitals
• Multiple Bonds

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Introduction

• Molecules have shapes and sizes that are defined
  by the angles and distances between nuclei of
  atoms.
• Shape, size, and strength and polarity of bonds
  determine properties of a substance.
• We start with Lewis structures to determine the
  number and types of bonds between atoms.

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Electron Domain

• Electron domain a region in which e-s will most
  likely be found.
• Bonding pair e- domain between two atoms.
• Non-bonding pair (lone pair) e- domain located
  mainly on one atom.
• An e- domain consists of a nonbonding pair, a
  single bond, or a multiple bond.

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Determining Number of Electron Domains
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• VSEPR Model Valence Shell Electron Pair
  Repulsion Model
• e-s are negatively charged, so they repel each
  other.
• VSEPR Model says that the best arrangement of a
  given number of e- domains is the one that
  minimizes the repulsions among them.

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VSEPR Model Using Balloons

• Electron domain geometry arrangement of e-
  domains around a central atom.

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• Electron-Domain Geometries

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Molecular Geometry

• The actual spacial arrangement of atoms.
• Molecular geometry is determined from e--domain
  geometry.
• To predict molecular shapes with VSEPR model,
  draw Lewis structure, count e--domains, then use
  the arrangement to determine molecular geometry.

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• Molecular Shapes from Electron-Domain Geometries

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• More Molecular Shapes from Electron-Domain
  Geometries

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Molecular Geometry Practice

• SeCl2

• Predict the molecular geometries

• O3

• SnCl3-

• CO32-

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Lone Pair Electrons on Trigonal Bipyramidal
Structures

• Nonbonding pairs always occupy equatorial
  positions on a trigonal bipyramidal structure.
• Can you explain why?

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Lone Pair Electrons on Octahedral Structures

• Nonbonding pairs always occupy the axial
  positions first.
• Why?

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Effect of Non-bonding Electrons Multiple Bonds

• Non-bonding pairs and multiple bonds exert
  greater repulsive forces on adjacent domains and
  tend to compress bond angles.
• Lone pair gt triple bond gt double bond gt single
  bond

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Bond Angle of Water

• Waters H-O-H bond angle is always 104.5º due to
  two lone electron pairs.

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Bond Angle of Methane and Ammonia

• NH3 has a smaller bond angle than methane due to
  the lone pair of electrons.

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Bond Dipoles

• Bond dipoles and dipole moments are vectors
  (magnitude direction).
• Overall dipole moment of a molecule is the sum of
  its bond dipoles.

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Polarity of Water

• If the water molecule were linear, water would
  NOT be polar.

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Polar or Nonpolar?

• BrCl
• Yes. All diatomics with polar bonds are polar
  molecules.
• SO2
• Yes. Bent molecule. Os more neg.
• NF3
• Yes. Trigonal bipyramidal geometry.
• BCl3
• No. Symmetry in trigonal planar geometry
• SF6
• No. Symmetry in octahedral arrangement.

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Molecular Shape Molecular Polarity

• Molecular polarity has a significant effect on
  physical and chemical properties.
• For a molecule with more than two atoms, dipole
  moment depends on polarities of individual bonds
  and the molecular geometry.

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Valence-Bond Theory

• Covalent bonds form when a valence atomic orbital
  of one atom merges with that of another atom.
• The orbitals overlap (share a region of space).
• The overlap allows two e-s of opposite spin to
  share common space.

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Formation of Bonds in H2

• The bond in H2 forms from the overlap of two 1s
  orbitals from two hydrogen atoms.

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Bonds From Orbital Overlap
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Hybridization

• To explain geometries, we assume atomic orbitals
  mix to form new hybrid orbitals.
• Hybridization the process of mixing and changing
  atomic orbitals as atoms approach to form bonds.

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Hybridization sp

• Linear arrangement of e- domains means sp
  hybridization.
• One s-orbital and one p-orbital hybridize to form
  two equivalent sp hybrid orbitals.

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Hybridization sp2

• Trigonal planar arrangement means sp2
  hybridization.
• One s-orbital and two p-orbitals hybridize to
  form three sp2 hybrids.

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Hybridization sp3

• Tetrahedral arrangement means sp3 hybridization.
• Four sp3 hybrids form from one s-orbital and
  three p-orbitals.

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Sigma (?) Bond

• Sigma (?) bond e- density concentrated
  symmetricaly around the line connecting the
  nuclei (internuclear axis).
• Single bonds are ? bonds.
• Can be made from s- or p-orbitals.
• Allows rotation at bond.

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Pi (?) Bond

• ? bond covalent bond in which there is a
  side-to-side overlap of p-orbitals.
• p-orbitals are perpendicular to internuclear
  axis.
• Overlap regions lie above and below internuclear
  axis.
• Less orbital overlap than in ? bonds, so ? bonds
  are weaker.
• Does NOT allow rotation around bond.

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Double Triple Bonds

• Double bonds consists of one ? bond and one ?
  bond.
• Triple bonds consist of one ? bond and two ?
  bonds.
• In a double bond, one set of p-orbitals overlap
  above and below the internuclear axis.
• In a triple bond, the second set of p-orbitals
  overlap in front of and behind the internuclear
  axis.

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Bonding in Ethylene
Ethylene is planar.
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Bonding in Acetylene
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Delocalized ? Bonding

• Resonance structures with ? bonds have
  delocalization of electrons.
• The electrons in these bonds extend over more
  than two bonded atoms.

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