Molecular Compounds

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Molecular Compounds
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Big Idea

• Covalent bonds form when atoms share electrons

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The Covalent Bond

• Atoms gain stability when they share electrons
  and form covalent bonds

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Molecular Compounds

• Substances consisting of molecules are covalently
  bonded
• Covalent bonds
• Made through electron sharing among atoms
• Electrons are NOT transferred
• Sharing allows substance to achieve stability
  (Noble gas configuration)
• Sharing forms molecules

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Molecules

• Are held together by the attraction of electrons
  of one atom and the nucleus of a second atom
• A single bond forms from a single pair of shared
  electrons
• Two pairs of electrons form a double bond
• Paired electrons have opposite spins and occupy
  less space than a pair of electrons surrounding
  only one atom
• Their bonds are flexible, somewhat like springs

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NaCl
Covalent compounds form molecules
Ionic compounds form a crystal lattice
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Bond Length and Strength of Bonds

• Average distance between nuclei of two bonded
  atoms
• Vary depending on other bonds present in the
  molecule
• As length increases, strength decreases.

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Bond Energy

• Amount of energy required to break bond to
  produce individual atoms (NOT ions)
• Best indicator of the strength of force of
  attraction
• Closer atoms require greater energy to separate
  them - the shorter the bond length, the greater
  the energy required to break it
• Large atoms have lower bond energies than small
  atoms

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What are Lewis Dot Structures?

• Structural formulas show the relative positions
  of atoms within a molecule
• Use the chemical symbol to represent the nucleus
  and inner energy levels
• Uses dots to represent valence electrons
• Types of bonds
• single bonds share 1 pair of electrons
• double bonds share 2 pairs of electrons
• triple bonds share 3 pairs of electrons

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Rules for Drawing Lewis Dot Structures

• See handout, Guide to Determining Molecular
  Shapes

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Single Covalent Bonds
When only one pair of electrons is shared, the
result is a single covalent bond.
The figure shows two hydrogen atoms forming a
hydrogen molecule with a single covalent bond,
resulting in an electron configuration like
helium.
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Single Covalent Bonds
Atoms in group 16 can share two electrons and
form two covalent bonds.
Water is formed from one oxygen with two hydrogen
atoms covalently bonded to it .
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Single Covalent Bonds
Atoms in group 15 form three single covalent
bonds, such as in ammonia.
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Single Covalent Bonds
Atoms of group 14 elements form four single
covalent bonds, such as in methane.
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Single Covalent Bonds
Sigma bonds are single covalent bonds.
Sigma bonds occur when the pair of shared
electrons is in an area centered between the two
atoms.
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Multiple Covalent Bonds
Double bonds form when two pairs of electrons are
shared between two atoms.
Triple bonds form when three pairs of electrons
are shared between two atoms.
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Molecular Shapes

• VSEPR Valence Shell Electron Pair Repulsion
  theory system for predicting molecular shape
  based on the idea that pairs of electrons orient
  themselves as far apart as possible
• Can only really be used with simple molecules

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Terms

• Structural formula indicates the spatial
  arrangement of atoms and bonds within a molecule
• Ligand an atom attached to the central atom
• Unshared pairs pairs of electrons that are not
  involved in covalent bonding, but instead belong
  exclusively to central atom

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unshared pairs ligands Molecular shape
0 2 Linear
1-2 Bent
0 3 Trigonal planar
1 Trigonal pyramidal
2 T shaped
0 4 Tetrahedral
1 See-saw
2 Square planar
0 5 Trigonal bipyramidal
1 Square pyramidal
0 6 octahedral
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Linear
Bent
Trigonal pyramidal
Trigonal Planar
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T-shaped
Tetrahedral
see-saw
Square planar
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Trigonal bipyramidal
Square pyramidal
Octahedral
http//chemlab.truman.edu/CHEM121Labs/MolecularMod
eling1.htm
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Polarity and Electronegativity

• A chemical bonds character is related to each
  atoms attraction for the electrons in the bond

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Polarity

• Electrons are not always shared equally in
  molecules
• Creates a partial charge within the molecule
• Atoms with uneven electronegativities share
  electrons unequally
• The greater the difference, the greater the
  polarity
• Polar having opposite ends one atom attracts
  electrons more strongly than the others
• Nonpolar doesnt have opposite ends electrons
  shared equally among bonding atoms

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Polar Covalent Bonds
Polar covalent bonds form when atoms pull on
electrons in a molecule unequally.
Electrons spend more time around one atom than
another resulting in partial charges at the ends
of the bond called a dipole.
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Examples

• CO2 Is a symmetrical molecule therefore it is
  nonpolar
• H2O H 2.20, O 3.44
• 3.44 2.20 1.24
• Water molecules are asymmetrical, so the molecule
  is polar covalent, with the electrons
  concentrating around the O atom (higher
  electronegativity)

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Differences in Electronegativity
Difference in Electronegativities Characteristic of Bond
gt 2.1 Mostly ionic (electrons transferred)
2.1 0.4 Polar Covalent (electrons unevenly shared)
lt0.4 Nonpolar Covalent (electrons equally shared)
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Diatomic Molecules

• A diatomic molecule is a molecule formed from two
  identical atoms
• The atoms join together because they are more
  stable that way than if they exist as single
  atoms
• Remember HOFBrINCl
• H2, O2, F2, Br2, I2, N2, and Cl2

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Why do atoms bond?
Diatomic molecules (H2, F2 for example) exist
because two-atom molecules are more stable than
single atoms.
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Molecular vs Empirical Formulas

• Molecular gives actual number of atoms of each
  element in a molecule
• Always a whole multiple of empirical formula
• Example glucose is C6H12O6 (6 x empirical
  formula)
• Empirical gives ratios of each element in each
  compound
• Subscripts always in lowest possible whole number
• Example glucose is CH2O

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To Determine Molecular Mass

• Divide the molar mass of the unknown compound by
  the molar mass of the empirical formula
• This lets you know what multiple is of the
  empirical formula the formula of the unknown
  molecular formula (empirical formula)n

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Example

• A compound was found to contain 65.45 C, 5.45
  H, and 29.09 O. The molar mass of the compound
  is 110.0 g/mol. What is the molecular formula?

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Example

• A colorless liquid composed of 46.68 N and
  53.32 O has a molar mass of 60.01 g/mol. What
  is the molecular formula?