Ch' 8 Covalent Bonding

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Ch. 8 Covalent Bonding
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Valence Electrons

• Are the electrons in the highest occupied energy
  level of an elements atoms
• The number of valence electrons largely
  determines the chemical properties of an element
• Usually only the valence electrons are used in
  chemical bonds

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Octet Rule

• In forming compounds atoms want to achieve a full
  octet (ns2np6)
• Metals tend to lose electrons
• Nonmetals tend to gain electrons

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Electron Dot Structures

• Show only the valence electrons
• Electrons are represented as dots
• One dot on each of the 4 sides-then paired up
  when necessary

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Covalent Bonds

• Form between 2 nonmetals
• Electrons are SHARED
• Mostly follow the octet rule
• Hydrogen only wants 2 electrons

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Molecule

• A neutral group of atoms joined by covalent bonds
• There is NO such thing as a molecule of NaCl or
  MgCl2 only collections of positively and
  negatively charged ions

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Diatomic Molecule

• A molecule consisting of 2 atoms

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Molecular Compounds

• A compound composed of molecules (CO2, H20)
• The molecules in a molecular compound are all the
  same
• Lower melting and boiling points than ionic
  compounds- most are gasses or liquids at room
  temperature
• Formed form 2 or more nonmetals

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Molecular Formulas

• Shows how many atoms of each element a molecule
  contains
• The subscripts are not necessarily the lowest
  whole number ratios
• Does not tell you about a molecules structure

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Single Covalent Bonds

• A bond in which 2 atoms share a pair of electrons
  
• In a covalent bond a pair of electrons is
  represented as a dash (structural Formula)
• Dashes are NEVER used to show ionic bonding

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Unshared Pair

• Also known as a lone pair or nonbonding pair- a
  pair of valence electrons that is NOT shared
  between atoms

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Double and Triple Bonds

• Double bonds involve the sharing of 2 pairs of
  electrons
• Triple bonds involve the sharing of 3 pairs of
  electrons

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Coordinate Covalent Bond

• A covalent bond where one atom contributes BOTH
  bonding electrons
• Once formed, is like any other covalent bond

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Polyatomic Ion

• Such as NH4 is a tightly bound group of atoms
  that has a positive or negative charge and
  behaves as a unit

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Bond Dissociation Energies

• The energy required to break the bond between two
  covalently bonded atoms

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Bond Dissociation Energy

• A large bond dissociation corresponds to a strong
  covalent bond

3 N-H single bonds 393 kJ/mol X 3
1179 kJ/mol
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Resonance Structure

• Is a structure that occurs when it is possible to
  draw 2 or more valid electron dot structures that
  have the same number of electron pairs for a
  molecule or ion

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Exceptions to the Octet Rule
Phosphorus Pentachloride
Boron Trifluoride
Sulfur Hexafluoride
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Exceptions to the Octet Rule

• The octet rule can NOT be satisfied in molecules
  whose total number of valence electrons is an odd
  number

N5 O6X2 17 valence electrons
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Exceptions to the Octet Rule

• An atom may acquires less than an octet of
  electrons
• B 3
• F 7 x 3
• 24 Valence Electrons

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Exceptions to the Octet Rule

• A few atoms (phosphorus and sulfur) expand their
  octet to include 10 or 12 electrons

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Molecular Orbitals

• When 2 atoms combine their atomic orbitals
  overlap to produce molecular orbitals that apply
  to the entire molecule

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Sigma (s) Bonds

• when 2 atomic orbitals combine to form a
  molecular orbital that is symmetrical around the
  axis connecting 2 atomic nuclei? single bond

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Pi (p) Bond

• The p atomic orbitals in a double or triple bond
  overlap side-by-side
• Weaker than sigma bonds

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VSEPR Theory

• Valence shell electron pair repulsion theory
• The repulsion between electron pairs causes
  molecular shapes to adjust so that
  valence-electron pairs are as far apart as
  possible

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Molecular Shapes
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Hybrid Orbitals

• Several atomic orbitals mix to form the same
  total number of equivalent hybrid orbitals

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Hybridization Involving Single Bonds
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Polar Covalent Bonds

• Not all electrons are shared 100 evenly
• In a polar molecule one end of the molecule is
  slightly positive, the other end is slightly
  negative
• A molecule that has 2 poles is called a dipole

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