Molecular Compounds

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Chapter 5

• Molecular Compounds

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Covalent Bonds

• Covalent Molecule A substance comprised of only
  nonmetals and metalloids
• Covalent Bond
• Formed by the sharing of electrons
• each atom within the bond achieves an octet (8)
  except hydrogen (2) and boron (6)
• Minimize the repulsions and maximize the
  attractions

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Covalent Bonds

• Diatomic molecules
• Br I N Cl H O F

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Naming Binary Molecular Compounds

• The atoms of a molecular formula are written in
  the order of the elements electronegativity
• Least electronegative atom will always be listed
  first.
• Electronegativity indicates how well an
  elements nuclei attract the electrons in a
  covalent bond

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The Periodic Table and Electronegativity
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Naming Covalent molecules

• Translate the elemental symbol into the elements
  name
• 1st element give its name
• 2nd element gets ending ide same as found in
  ionic compounds
• Because electrons are shared
• Indicate how many of each atom are present using
  the prefix system
• All elements will receive a prefix except if
  there is only 1 of the first element

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Naming Binary Molecular Componds
The prefix mono- is omitted for the first element.
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Problem

• What is the name of PCl3? 
• a.  phosphorus chloride
• b.  phosphoric chloride
• c.  phosphorus trichlorate
• d.  trichlorophosphide
• e.  phosphorus trichloride

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Problem

• The compound, P4S10, is used in the manufacture
  of safety matches. What is its name? 
• a.  phosphorus sulfide
• b.  phosphoric sulfide
• c.  phosphorus decasulfide
• d.  tetraphosphorus decasulfide
• e.  phosphorus sulfide

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Molecular Formulas

• Translate the words

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Covalent Bonds and the Periodic Table

• Covalent bonds form between nonmetals
• Achieve an octet (noble gas configuration)
• Using electron dot symbols build
• H2, H2O, CH4, O2, N2, HCN, CO2
• Least electronegative atom is central (except H)

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Covalent Bonds and the Periodic Table

• Elements follow octet rule
• H gets 2 electrons 1 bond
• B has 3 valence electrons can only form 3 bonds
• Period 3 elements and higher can form more than
  an octet because of the empty d-orbitals

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Covalent Bonds and the Periodic Table
Number of bonds formed to achieve octet. Numbers
in parentheses indicate possible numbers of bonds
that result in exceptions to the octet rule.
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Multiple Covalent Bonds

• Single bond A covalent bond formed by sharing
  one electron pair.
• Double bond A covalent bond formed by sharing
  two electron pairs.
• Triple bond A covalent bond formed by sharing
  three electron pairs.

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Molecular Formulas and Lewis Structures

• Molecular formula A formula that shows the
  numbers and kinds of atoms in one molecule of a
  compound.
• Structural formula A molecular representation
  that shows the connections among atoms by using
  lines to represent covalent bonds.
• Lewis structure A molecular representation that
  shows both the connections among atoms and the
  locations of lone-pair valence electrons.

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Molecular Formulas and Lewis Structures

• The oxygen atom in H2O
• shares 2 bonding pairs of electrons with two
  hydrogen atoms
• Has 2 other pairs of valence electrons that are
  not shared in bonds lone pairs

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Drawing Lewis Structures

• Sum up the valence electrons for all atoms in the
  molecule
• Identify the atom in the lowest group (except
  H) Least electronegative atom
• If both atoms in same group identify the one in
  the highest numbered period.
• Write this symbol as the central atom surrounded
  by all other atoms

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Drawing Lewis Structures

• Place a single bond (-, representing two
  electrons) between all external atoms and the
  central atom
• Add up the electrons in the bonds and subtract
  this from the total valence electrons

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Drawing Lewis Structures

• Place all other electrons around the outer atoms
  (in pairs) first giving them an octet (except H)
• Most electronegative atom first
• Place all remaining electrons around the central
  atom as lone pairs
• Check to see if all atoms have an octet
• Yes you are done
• No make multiple bonds

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Drawing Lewis Structures

• H, C, N, O, and halogen atoms usually maintain
  consistent bonding patterns
• H forms one covalent bond.
• C forms four covalent bonds.
• N forms three covalent bonds and has one lone
  pair of electrons.
• O forms two covalent bonds and has two lone pairs
  of electrons.
• Halogens form one covalent bond and have three
  lone pairs of electrons.

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The Shapes of the Molecules

• Molecular shapes can be predicted by noting how
  many bonds and electron pairs surround individual
  atoms and applying what is called the
  valence-shell electron-pair repulsion (VSEPR)
  model.
• VSEPR model - the negatively charged clouds of
  electrons in bonds and lone pairs repel each
  other, and keep as far apart as possible.

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The Shapes of the Molecules

• There are three step to applying the VSEPR model
• Step 1 Draw a Lewis structure of the molecule,
  and identify the atom whose geometry is of
  interest.
• Step 2 Count the number of electron charge
  clouds surrounding the atom of interest.
• Step 3 Predict molecular shape by assuming that
  the charge clouds orient in space so that they
  are as far away from one another as possible.

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The Shapes of the Molecules

• Linear molecules have bond angles of 180.
• Planar triangular molecules have bond angles of
  120.
• Tetrahedral molecules have bond angles of 109.5.

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Polar Covalent Bonds and Electronegativity

• As a rule of thumb
• electronegativity differences of less than 0.5 -
  nonpolar covalent bonds
• differences up to 1.9 indicate polar covalent
  bonds
• differences of 2 or more indicate ionic bonds.

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Problem

• Select the most polar bond amongst the
  following. 
• a.  C-O
• b.  Si-F
• c.  Cl-F
• d.  C-F
• e.  C-I

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Polar Molecules

• If the bonds in a molecule have a polarity then
  depending on the shape of the molecule, the
  molecule itself can be polar
• Guaranteed nonpolar molecules
• Linear, trigonal planar and tetrahedral if all
  peripheral atoms are identical

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Characteristics of Molecular Compounds