Covalent Bonding

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6-2

• Covalent Bonding

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• MOLECULE neutral group of atoms held together by
  covalent bonds
• MOLECULAR COMPOUND a cmpd that is made of
  molecules
• MOLECULAR FORMULA shows the kinds s of atoms
  making up a molecule i.e. H2O
• DIATOMIC MOLECULE molecule containing only 2
  atoms i.e. H2

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Why would nature favor forming a covalent bond?
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• Attraction nucleus of one atom to the electron
  cloud of the other atom
• Repulsion both nuclei repelas do both electron
  clouds

• As atoms approach, attraction increases and
  potential E goes down
• Beyond a certain point, repulsion increases and E
  goes up

? bottom of valley on E curve is where there is a
balance between attraction and repulsion
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Characteristics of the Covalent Bond

• Bond Length average distance between 2 bonded
  atoms (min potential E)

In the H2 molecule, the 1s orbitals overlap and
result in a increased electron density between
the nuclei.
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Bond energy

• Energy required to break a chemical bond and form
  neutral isolated atoms
• Measured in kJ/mol (kilojoules/mole)
• Positive value
• Bond energy and bond length values will vary
  depending on what atoms an element is bonded
  toavg. values found on tables P.168)

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Octet Rule

• Chemical cmpds tend to form so that each atom, by
  gaining, losing or sharing electrons, has an
  octet (8) of electrons in its highest (outermost)
  energy level
• Example of exceptions
• H only has 2 valence e-s to reach stability
• B only has 6 valence e-s to reach stability
• Some elements have gt 8 (occurs when atom bonds
  with highly electroneg. elements some d e-s
  are involved in bonding)

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Electron Dot Diagrams

• Electron configuration notation in which only the
  valence electrons of an atom of a particular
  element are shown, indicated by dots placed
  around the elements symbol
• F 1s22s22p5

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other dot diagram format

• Mg Ne3s2
• paired electrons are shown as a pair

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Lewis Structures

• Formulas in which.
• atomic symbols represent nuclei and inner-shell
  electrons
• Dot-pairs or dashed between 2 atoms represent
  electron pairs in covalent bonds
• Dots adjacent to only one atomic symbol represent
  unshared electrons

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Lewis structure
Structural Formula indicates The kind, number,
arrangement, and bonds..but NOT the
unshared Pairs of the atoms in a molecule
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Lewis structure
Single bond covalent bond produced by the
sharing on ONE pair of electrons between 2 atoms
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How do you draw a Lewis Structure? Ex Carbon
Tetrachloride (CCl4)

• 1. Determine the type and number of atoms in the
  molecule
• One C atom and 4 chlorine atoms
• 2. Write the electron dot notation for each type
  of atom in the molecule
• C has 4 dots around it (one on each side).
  For 4 valence e-s
• Each Cl has 7 dots for its 7 valence electrons

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• 3. determine the total number of valence
  electrons in the atoms to be combined.
• C 1 x 4 e - 4 electrons
• Cl 4 x 7 e- 28 electrons
• Total 32 electrons
• 4. Arrange the atoms to form a skeleton structure
  for the molecule. If C is present, it is the
  central atom. Otherwise , the least
  electronegative atom is central (except H, it is
  never central). Then connect the atoms by
  electron-pair bonds. Add unshared electrons.

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• 5. count the number of electrons in the structure
  to be sure that the number of valence electrons
  used equals the number available. Be sure at all
  the atoms besides H have an octet.

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Multiple Covalent Bonds

• DOUBLE BOND
• sharing of TWO pairs of electrons between 2 atoms
• TRIPLE BOND
• sharing of THREE pairs of electrons between 2
  atoms
• Bond Energies
• triple gt double gt single
• ..triple bonds are stronger and shorter

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WE will do an example on the board!!!
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Resonance Structures

• bonding in molecules or ions that cannot be
  correctly represented by a single Lewis structure

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Draw the resonance structure for sulfur trioxide,
SO3

• WE will do this on the board also!!

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Draw the resonance structure for sulfur trioxide,
SO3

• S 6 e-
• O 3 x 6 e-
• Total 24 e-

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Covalent-network Bonding

• Covalent bonding exists
• throughout a large network
• of atomswill be
• discussed in chapter 12
• i.e. carbon atoms
• in a diamond

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HOMEWORK

• Do Section Review on p. 175