8'1 Chemical Bonds, Lewis Symbols, and the Octet Rule

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8.1 Chemical Bonds, Lewis Symbols, and the Octet
Rule

• Chemical bond - attractive force between atoms or
  ions
• Ionic bond - electrostatic force between
  oppositely-charges ions results from the
  transfer of electrons from a metal to a nonmetal.
• Covalent bond results from sharing electrons
  between the atoms usually found between
  nonmetals.
• Polar covalent unequal sharing of electrons
• Metallic bond attractive force holding pure
  metals together.

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Lewis Symbols

• A pictorial representation of the valence
  electrons
• Electrons are represent as dots around the symbol
  for the element.

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The Octet Rule

• All noble gases except He have an s2p6
  configuration.
• Octet rule atoms tend to gain, lose, or share
  electrons until they are surrounded by 8 valence
  electrons (4 electron pairs).
• C, N, O, and F always obey the octet rule
• Caution there are many exceptions to the octet
  rule (section 8.7).

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8.3 Covalent Bonding

• Covalent bonds can be represented by the Lewis
  symbols of the elements
• In Lewis structures, each pair of electrons in a
  bond is represented by a single line

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Multiple Bonds

• It is possible for more than one pair of
  electrons to be shared between two atoms, i.e.
  multiple bonds.
• One shared pair of electrons single bond (e.g.
  H-H)
• Two shared pairs of electrons double bond (e.g.
  OO)
• Three shared pairs of electrons triple bond
  (e.g. NN)
• Bond order number of bonds between two atoms
• Generally, bond strength increases and bond
  distance decreases as bond order increases.

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8.5 Drawing Lewis Structures

• Add up the valence electrons.
• ( valence e of atoms e available for
  molecule)
• Ionic charges charge - fewer e - charge -
  more e
• Write symbols for the atoms and connect with
  single bonds. Geometry doesnt matter at this
  point.
• Complete the octets of the outer atoms.
• Place leftover electrons (in pairs) on the
  central atom.
• If there are not enough electrons to give the
  central atom an octet, move electrons from outer
  atoms to form multiple bonds.

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8.5 Drawing Lewis Structures

• Formal Charges
• There may be more than one valid Lewis Structure
  for a given molecule.
• Formal charges are used to determine the most
  reasonable structure.
• Calculate a formal charge (FC) for each atom
• FC ( valence e) - ( e belonging to atom)
• Best structure? The one with lowest formal
  charges and one with the most negative charges on
  the most electronegative atoms.

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8.7 Exceptions to the Octet Rule

• There are three classes of exceptions to the
  octet rule.
• Molecules with
• an odd number of electrons
• one or more atoms with less than an octet
• one or more atoms with more than an octet
• Odd Number of Electrons (radicals)
• Molecules such as ClO2, NO, and NO2 have an odd
  number of electrons. (will not see these on
  worksheet)

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8.7 Exceptions to the Octet Rule

• Less than an Octet
• Relatively rare.
• Typical for elements of Groups 1A, 2A, and 3A.
• H 2 electrons (duet rule)
• Be 4 electrons
• B 6 electrons
• Formal charges indicate that the Lewis structure
  with an incomplete octet is more important than
  the ones with double bonds.

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8.7 Exceptions to the Octet Rule

• More than an Octet
• Very common for central atom, rare for outer
  atoms
• Atoms from the 3rd period onwards can accommodate
  more than an octet, e.g. P (10), S (12), Cl (14),
  Xe (16)
• How? Beyond the third period, the d orbitals are
  low enough in energy to participate in bonding
  and accept the extra electron density.

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BH3
CH4
PCl5
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9.1 Molecular Shapes

• Lewis structures show which atoms are physically
  connected electron domains
• The shape of a molecule is determined by its bond
  angles.
• CCl4 experimentally find all Cl-C-Cl bond angles
  are 109.5?.
• Therefore, the molecule cannot be planar.
• All Cl atoms are located at the vertices of a
  tetrahedron with the C at its center.

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9.2 The VSEPR Model

• Valence Shell Electron Pair Repulsion (VSEPR)
  theory.
• Works by positioning electron domains as far
  apart as possible to minimize electron repulsion
• Each region of electrons about central atom is an
  electron domain.
• Single bond one domain
• Lone pair one domain
• Double or triple bond one domain
• Total number of electron domains predicts
  electronic geometry (or electron-domain geometry)
• The arrangement of atoms in space is the
  molecular geometry (3D shape)

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Octahehron
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9.2 The VSEPR Model

• When determining the electronic geometry, all
  electrons (lone pairs and bonding pairs) are
  considered.
• When naming the molecular geometry, focus only on
  the positions of the atoms.

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9.2 The VSEPR Model

• To determine the geometry
• Draw the Lewis structure.
• Count the total number of electron domains around
  the central atom which gives electronic geometry.
• Arrange the electron domains in a geometry which
  minimizes e--e- repulsion counting multiple bonds
  as one bonding pair.
• Assign molecular geometry.
• Include multiple bonds in VSEPR structure, but
  lone pairs not necessary.

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BH3
CH4
PCl5
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H2O
CO32-
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• The Effect of Lone Pairs and Multiple Bonds on
  Bond Angles
• Since electrons in a bond are attracted by two
  nuclei, they do not repel as much as lone pairs.
• Therefore, the bond angle decreases as the number
  of lone pairs increase.
• Multiple bonds repel more than single bonds, and
  the same affect is seen.

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Trigonal Bipyramidal Geometry

• To minimize e-- e- repulsion, lone pairs are
  always placed in equatorial positions.

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