Chapter 10 Molecular Structure and Bonding Theories

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Chapter 10 Molecular Structure and Bonding
Theories
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VSEPR

• Valence-Shell Electron-Pair Repulsion Model
  (VSEPR) predicts shape from Lewis Structures.
• VSEPR Rule 1 A molecule has a shape that
  minimizes electrostatic repulsions between
  valence-shell electron pairs.
• Minimum repulsion results when the electron pairs
  are as far apart as possible.

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Steric Number

• Steric number (number of lone pairs on central
  atom) (number of atoms bonded to central atom)
• The steric number is determined from the Lewis
  structure.
• Steric number determines the bonded-atom
  lone-pair arrangement, the shape that maximizes
  the distances between the valence-shell electron
  pairs.

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Geometric Arrangements
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Geometric Arrangements
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Steric Number 2

• In the Lewis structure of BeCl2,
• beryllium has two bonded atoms and no lone
  pairs, steric number 2.
• A linear geometry places the two pairs of
  electrons on the central beryllium atom as far
  apart as possible.

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Molecules with Multiple Bonds

• The Lewis structure of HCN (H-Cº N) shows that
  the carbon atom is bonded to two atoms and has no
  lone pairs, steric number 2.
• The bonded-atom lone-pair arrangement is linear.
• The number of bonded atoms, not the number of
  bonds, determines the steric number.

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Steric Number 3

• The Lewis structure of BF3
• shows the boron atom has a steric number 3
  the bonded-atom lone-pair arrangement is trigonal
  planar.

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Steric Number 4

• The Lewis structure of CH4
• shows the carbon atom has a steric number 4
  the bonded-atom lone-pair arrangement is
  tetrahedral.

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Steric Number 5

• The phosphorus atom in PF5 has a steric number
  5 the bonded-atom lone-pair arrangement is
  trigonal bipyramidal.

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Steric Number 6

• The sulfur atom in SF6 has a steric number 6
  the bonded-atom lone-pair arrangement is
  octahedral.

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Central Atoms with Lone Pairs

• The Lewis structure of H2O is
• Steric number 4, 2 bonded atoms and 2 lone
  pairs.
• The bonded-atom lone-pair arrangement is
  tetrahedral.

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Molecular Shape of H2O

• Molecular shape is the arrangement of the atoms
  in a species.
• The bonded-atom lone-pair arrangement of H2O is
  tetrahedral (top) the molecular shape is bent or
  V-shaped (bottom).

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Molecular Shape of NH3

• What is the electron pair geometry and molecular
  shape of NH3?

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Electron Pair Repulsions

• The measured bond angle in H2O (104.5o) is
  smaller than the predicted angle (109.5o)
• Explanation repulsions vary lone pair-lone pair
  gt lone pair-bonding pair gt bonding pair-bonding
  pair

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Location of Lone Pair in SF4
Two structures are possible

• The favored structure for a trigonal bipyramid
  minimizes 90o lone pair interactions the one on
  the right.

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Lone Pairs in Trigonal Bipyramids

• Lone pairs always occupy the equatorial positions
  in a trigonal bipyramid so that lone pair-lone
  pair repulsions are oriented at 120o.

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Location of Lone Pairs in XeF4

• The structure on right has no 90o lone pair-lone
  pair interactions and is favored.

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Test Your Skill

• What is the steric number, the bonded-atom
  lone-pair arrangement, and the molecular shape of
  ClF3?

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Multiple Central Atoms

• The geometry of each central atom is determined
  separately.
• The CH3 carbon in CH3CN has tetrahedral geometry
  and the other carbon has linear geometry.

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Shapes of Molecules

• What are the bonded-atom lone-pair arrangements
  and the shapes about each central atom in NH2SH?
• Draw the Lewis structure.
• The bonded-atom lone-pair arrangements of both
  are tetrahedral, the nitrogen shape is trigonal
  pyramidal and sulfur is V shaped.

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Overall Shape of C2H4

• Ethylene, C2H4 , could be planar (left) or
  nonplanar (right). The VSEPR model does not
  predict which is preferred.

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Polarity of Molecules

• The bond dipoles in CO2 cancel because the linear
  shape orients the equal magnitude bond dipoles in
  exactly opposite directions.

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Polarity of Molecules

• The bond dipoles do not cancel in COSe they are
  oriented in the same direction and are of unequal
  length. They do not cancel in OF2 because the
  V-shape of the molecule does not orient them in
  opposite directions.

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Polarity of Molecules

• The bond dipoles in BCl3 and CCl4 cancel because
  of the regular shape and equal magnitude.

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Polarity of Molecules

• The bond dipoles in BCl2F and CHCl3 do not cancel
  because they are not of the same magnitude.

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Test Your Skill

• Are the following molecules polar or nonpolar
  H2S, SiF4, CH2Cl2?

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Valence Bond Theory

• Valence bond theory describes bonds as being
  formed by overlap of partially filled valence
  orbitals.

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Test Your Skill

• Identify the orbitals that form the bond in HCl.

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Bonding in NH3

• The observed bond angles of 107.5o in NH3 are not
  consistent with the angles of 90o expected if
  the bonds formed from N 2p orbitals.

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Hybrid Orbitals

• Hybrid orbitals are orbitals obtained by mixing
  two or more atomic orbitals on the same central
  atom.
• Appropriate hybrid orbitals formed by mixing one
  s and xp atomic orbitals make bonds at either
  180o (x 1), 120o (x 2), or 109.5o (x 3).

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Analogy for Hybrid Orbitals
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sp Hybrid Orbitals

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Shape of Hybrid Orbitals

• For clarity, hybrid orbitals are pictured as
  elongated with the small lobe omitted.

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Bonding in BeCl2

• The bonds in BeCl2 arise from the overlap of two
  sp hybrid orbitals on the beryllium atom with the
  3p orbitals on the two chlorine atoms.

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sp2 Hybrid Orbitals

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Bonding in BF3

• The bonds in BF3 arise from the overlap of three
  sp2 hybrid orbitals on the boron atom with 2p
  orbitals on the three fluorine atoms.

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sp3 Hybrid Orbitals

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Bonding in CH4

• The bonds in CH4 arise from the overlap of four
  sp3 hybrid orbitals on the carbon atom with 1s
  orbitals on the four hydrogen atoms.

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Lone Pairs and Hybrid Orbitals

• Hybrid orbitals can hold lone pairs as well as
  make bonds.

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Hybridization with d Orbitals

• Hybrid orbitals of central atoms with steric
  numbers of 5 or 6 involve d orbitals.

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Hybrid Orbitals
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Test Your Skill

• Identify the hybrid orbitals on the central atoms
  in SiH4 and HCN.

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Types of Bonds Sigma Bonds

• Sigma bonds (s) the shared pair of electrons is
  symmetric about the line joining the two nuclei
  of the bonded atoms.

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Bonding in C2H4

• The C-C sigma bond in C2H4 arises from overlap of
  sp2 hybrid orbitals and the four C-H sigma bonds
  from overlap sp2 hybrid orbitals on C with 1s
  orbitals on H.
• The second C-C bond forms from sideways overlap
  of p orbitals.

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Types of Bonds Pi Bonds

• Pi bonds (p) places electron density above and
  below the line joining the bonded atoms they
  form by sideways overlap of p orbitals.

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Bonding in C2H4

• The double bond in C2H4 is one sigma bond and one
  pi bond each bond is of similar strength.

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Proof of Pi Bonds Shape of C2H4

• C2H4 is planar (A) because pi overlap is at a
  maximum. Rotation of one end by 90o (B) reduces
  pi overlap to zero.

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Triple Bonds

• The triple bond in C2H2 is one sigma bond and two
  pi bonds between the sp hybridized carbon atoms.

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Sigma Bonds in Benzene

• Each carbon atom in benzene, C6H6, forms three
  sigma bonds with sp2 hybrid orbitals.

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Pi Bonds in Benzene

• The remaining p orbital on each carbon atom (top)
  overlap to form three pi bonds.

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Test Your Skill

• Describe the bonds made by the carbon atom in HCN.

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Molecular Orbital Theory

• Molecular orbital theory is a model that combines
  atomic orbitals to form new molecular orbitals
  that are shared over the entire molecule.
• A bonding molecular orbital concentrates electron
  density between atoms in a molecule.
• An antibonding molecular orbital reduces electron
  density between atoms in a molecule.

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Hydrogen Molecule

• Addition of the 1s orbitals of two H atoms forms
  a sigma bonding molecular orbital and subtraction
  forms a sigma antibonding molecular orbital,
  indicated with a symbol.

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Molecular Orbital Diagram H2

• Bonding molecular orbitals are more stable and
  antibonding molecular orbitals are less stable
  than the atomic orbital that are combined.

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Bond Order

• Bond order 1/2 number of electrons in bonding
  orbital - number of electrons in antibonding
  orbitals
• Bond order in H2 1/2 2 - 0 1

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Molecular Orbital Diagram He2

• Bond order in He2 1/2 2 - 2 0 the molecule
  does not form.

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Sigma Molecular Orbitals from p

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Pi Molecular Orbitals from p

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MO Diagram Second-Period Diatomics

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Molecular Orbital Diagram N2

• The electron configuration is(s2s)2(s2s)2(p2p)4
  (s2p)2.
• The bond order in N2 is three and there are no
  unpaired electrons.
• Lewis theory (Nº N) predicts the same result.

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Molecular Orbital Diagram Be2

• The electron configuration is (s2s)2(s2s)2 .
  
• Bond order in Be2 is zero and the molecule does
  not exist.

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Molecular Orbital Diagram for O2

• Draw the molecular orbital diagram of O2. What
  is the electron configuration, the bond order and
  how many unpaired electrons are present?

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Test Your Skill

• Draw the molecular orbital diagram of B2. What
  is the electron configuration, the bond order and
  number of unpaired electrons?