Ch. 8�Covalent Bonding by Z5GGnBMW

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									         Ch. 8—Covalent Bonding
• 8.1—Molecular Compounds
 covalent bond, diatomic molecule, molecular formula

• 8.2—The Nature of Covalent Bonding
 single, double, and triple covalent bonds, structural formula, unshared
 pair , coordinate covalent bonds (skip bond dissociation, and resonance)

• 8.3—Bonding Theories (skip all—need to
  know for chem AP though)
• 8.4—Polar Bonds and Molecules
 bond polarity, electronegativity, polar molecules, Van der Waals forces
 (dipole interactions and dispersion forces, hydrogen bonds, network solid)
8.1
        8.1 Molecular Compounds

 These toy models are made
 from circular pieces joined
 together in units by sticks.
 Atoms can also be arranged
 in different ways to make a
 variety of products.
              Ionic vs. Covalent
• Ionic bonds- the electrostatic forces that hold
  ions together; cations and anions combine to
  form a neutral compound (metal + nonmetal)
     ex. NaCl, (NH4)2O
• Covalent bonds- atoms held together by
  sharing electrons (nonmetal + nonmetal)
     ex. H2O, NH3
     8.1
    Molecules and Molecular Compounds
    A molecule is a neutral group of atoms joined
    together by covalent bonds. Air contains oxygen
    molecules.

•   In nature, matter takes many forms. The noble gases, including
    helium and neon, are monatomic. That means they exist as single
    atoms.

•   A diatomic molecule is a molecule consisting of two atoms. An
    oxygen molecule is a diatomic molecule. There are 7 diatomic
    elements: F2, Cl2, Br2, I2, H2, N2, and O2

A compound composed of molecules is                   Molecular compounds tend
called a molecular compound. Water and                to have relatively lower
carbon monoxide are molecular                         melting and boiling points
compounds.                                            than ionic compounds.
8.1
       Molecules and Molecular
            Compounds




A molecular formula is the chemical formula of a molecular compound.
A molecular formula shows how many atoms of each element a molecule
contains.
8.1
Molecules and Molecular Changes
 Ethane, a
 component
 of natural
 gas, is also
 a molecular
 compound.
8.1
      Molecular Formulas
            8.1 Section Quiz.
1. Compared to ionic compounds, molecular
   compounds tend to have relatively
  a. low melting points and high boiling points.
  b. low melting points and low boiling points.
  c. high melting points and high boiling points.
  d. high melting points and low boiling points.
            8.1 Section Quiz

2. A molecular compound usually consists of
   a. two metal atoms and a nonmetal atom.
   b. two nonmetal atoms and a metal atom.
   c. two or more metal atoms.
   d. two or more nonmetal atoms.
             8.1 Section Quiz

3. A molecular formula shows
   a. how many atoms of each element a molecule contains.
   b. a molecule's structure.
   c. which atoms are bonded together.
   d. how atoms are arranged in space.
8.2
 8.2 The Nature of Covalent Bonding

 The colors in this map
 indicate the concentrations
 of ozone in various parts of
 Earth’s atmosphere.
 Oxygen atoms can join in
 pairs to form the oxygen
 you breathe and can also
 join in groups of three
 oxygen atoms to form
 ozone.
The Octet Rule in Covalent Bonding
   In covalent bonds, electron sharing usually
   occurs so that atoms attain the electron
   configurations of noble gases.
 Ex.    H2—each hydrogen shares its
       valence electron to fill the outer
       orbital with 2 electrons (first energy
       level)
 Ex.   CH4—each hydrogen shares its
       valence electron with carbon to fill
       carbon’s outer shell with 8
       electrons and each hydrogen has
       2 electrons
 Ex.   O2—each oxygen shares two
       valence electrons with each other
       to fill both outer shells; this is a
       double covalent bond
   8.2
                Single Covalent Bonds




An electron dot structure such as H:H
represents the shared pair of electrons
of the covalent bond by two dots
whereas a structural formula such as
H—H represents the covalent bonds
by dashes and shows the arrangement
of covalently bonded atoms.
8.2
       Single Covalent Bonds
  The hydrogen and oxygen atoms attain noble-gas
  configurations by sharing electrons.
  8.2
                Single Covalent Bonds
A pair of valence electrons
                               Methane has no unshared
that is not shared between     pairs of electrons.
atoms is called an unshared
pair, also known as a lone
pair or a nonbonding pair.




The ammonia molecule has one
unshared pair of electrons.
8.1
8.1
8.2
Double and Triple Covalent Bonds
  Atoms form double or triple covalent bonds if they
  can attain a noble gas structure by sharing two
  pairs or three pairs of electrons.
       A bond that involves two shared pairs of electrons is a
       double covalent bond.

       A bond formed by sharing three pairs of electrons is a
       triple covalent bond.


 Each oxygen atom has one
 unshared pair of electrons.

 Nitrogen forms a triple covalent
 bond.
  8.2
  Another Double Covalent Bond Example
Carbon dioxide gas is soluble in
water and is used to carbonate     Carbon dioxide is an example of a
many beverages. A carbon dioxide   triatomic molecule (containing 3
molecule has two carbon-oxygen     atoms).
double bonds.
8.2
Double and Triple Covalent Bonds
8.2
      Coordinate Covalent Bonds
      A coordinate covalent bond is a covalent bond in which
        one atom contributes to both bonding electrons.
      • In a coordinate covalent bond, the shared electron pair
        comes from one of the bonding atoms
      • Arrows are used to point from the atom donating the
        pair of electrons to the atom receiving them

                           The polyatomic ammonium ion (NH4),
                           present in ammonium sulfate, is an
                           important component of fertilizer for field
                           crops, home gardens, and potted plants.
                           The ammonium ion consists of atoms
                           joined by covalent bonds, including a
                           coordinate covalent bond.
8.2
      Coordinate Covalent Bonds
  8.2
       Exceptions to the Octet Rule
                                             NO2 is produced naturally by
The octet rule cannot be satisfied in        lightning strikes.
molecules whose total number of valence
electrons is an odd number. There are also
molecules in which an atom has fewer, or
more, than a complete octet of valence
electrons.




                                                 Two electron dot structures
                                                 can be drawn for the NO2
                                                 molecule.
                   8.2 Section Quiz.
1. In covalent bonding, atoms attain the
  configuration of noble gases by
   a.   losing electrons.
   b.   gaining electrons.
   c.   transferring electrons.
   d.   sharing electrons.
             8.2 Section Quiz
2. Electron dot diagrams are superior to molecular
  formulas in that they
   a. show which electrons are shared.
   b. indicate the number of each kind of atom in the
     molecule.
   c. show the arrangement of atoms in the molecule.
   d. are easier to write or draw.
              8.2 Section Quiz
3. Which of the following molecules would contain
  a bond formed when atoms share three pairs of
  electrons?
   a.   Se2
   b.   As2
   c.   Br2
   d.   Te2
8.4
      8.4 Polar Bonds and Molecules

Snow covers approximately 23 percent of
Earth’s surface. Each individual snowflake is
formed from as many as 100 snow crystals.
The polar bonds in water molecules influence
the distinctive geometry of snowflakes.
8.4
                     Bond Polarity
When the atoms in a bond pull equally (as occurs when identical atoms
are bonded), the bonding electrons are shared equally, and the bond is a
nonpolar covalent bond.
The bonding pairs of electrons in covalent bonds are pulled by the nuclei.
A polar covalent bond, known also as a polar bond, is a covalent bond
between atoms in which the electrons are shared unequally.
8.4
                Bond Polarity
The more electronegative atom attracts electrons
more strongly and gains a slightly negative charge.
The less electronegative atom has a slightly positive
charge.
                                         The chlorine
                                         atom attracts
                                         the electron
                                         cloud more
                                         than the
                                         hydrogen
                                         atom does.
 8.4
                      Bond Polarity




Electronegativity values increase across the periodic table from bottom
left to top right; subtract elements electronegativity values to determine
if a compound is more likely to form ionically or covalently
 8.4
  Attractions Between Molecules
 Intermolecular attractions are weaker than either ionic or
 covalent bonds. These attractions are responsible for
 determining whether a molecular compound is a gas, a
 liquid, or a solid at a given temperature.

Van der Waals Forces
       The two weakest attractions between molecules are
       collectively called van der Waals forces, named after
       the Dutch chemist Johannes van der Waals (1837–
       1923). Dipole interactions occur when
                                               Dispersion forces are
               polar molecules are attracted   caused by the motion of
               to one another                  electrons
      8.4
      Attractions Between Molecules
  Dipole interactions occur when
                                              Dispersion forces, the weakest
  polar molecules are attracted to one        of all molecular interactions, are
  another.                                    caused by the motion of
                                              electrons.
                                              The strength of dispersion forces
                                              generally increases as the
                                              number of electrons in a molecule
                                              increases.


                                              Hydrogen bonds are attractive
                                              forces in which a hydrogen
                                              covalently bonded to a very
In a polar molecule, one end of the           electronegative atom is also
molecule is slightly negative and the other   weakly bonded to an unshared
end is slightly positive.                     electron pair of another
                                              electronegative atom.
A molecule that has two poles is called a
dipolar molecule, or dipole.
8.4
Attractions Between Molecules
Hydrogen Bonding in Water




                            The relatively strong attractive
                            forces between water molecules
                            cause the water to form small
                            drops on a waxy surface.
  8.4
   Intermolecular Attractions and
        Molecular Properties
Network solids (or network crystals) are solids in which all of the
atoms are covalently bonded to each other.
Network solids consist of molecules that do not melt until the
temperature reaches 1000°C or higher, or they decompose
without melting at all.
Melting a network solid would require breaking covalent bonds
throughout the solid.

Examples are diamonds and silicon carbide (grindstones)
8.4
 Intermolecular Attractions and
      Molecular Properties
Diamond is an example of a network solid. Diamond does not
melt. It vaporizes to a gas at 3500°C or above. Silicon Carbide
is a network solid. It has a melting point of about 2700°C.
     Intermolecular Attractions and
          Molecular Properties




molecule, molecular formula, molecular compound, covalent bond
formula unit, chemical formula, ionic compound, ionic bond
                 8.4 Section Quiz.
1.In a molecule, the atom with the largest electronegativity
     value
   a. repels electrons more strongly and aquires a slightly negative
         charge.
   b. repels electrons more strongly and aquires a slightly positive
         charge.
   c. attracts electrons more strongly and aquires a slightly positive
         charge.
   d. attracts electrons more strongly and acquires a slightly negative
         charge.
                8.4 Section Quiz.
2. When polar molecules are placed between oppositely
  charged plates, the negative
   a. molecules stick to the positive plates.
   b. molecules stick to the negative plates.
   c. ends of the molecules turn toward the positive plates.
   d. ends of the molecules turn toward the negative plates.
            8.4 Section Quiz.
3. Which of the following bond types is the weakest?
    a. ionic bond
    b. Van der Waals force
    c. covalent bond
    d. hydrogen bond


Ionic bond (intramolecular) , covalent bond
(intramolecular), hydrogen bonds
(intermolecular), Van der Waals forces
(dipole then dispersion) in order of
decreasing strength
9.3 Naming and Writing Formulas for
      Molecular Compounds

What does a prefix in the name of a binary molecular
compound tell you about the compound’s composition?
A prefix in the name of a binary molecular
compound tells how many atoms of an element are
present in each molecule of the compound.
9.3
      Naming Binary Molecular Compounds
Carbon and oxygen combine to form carbon monoxide
(CO) and carbon dioxide (CO2), but these two invisible
gases are very different.




 Sitting in a room with small amounts of CO2 in the air
 would not present any problems. If the same amount
 of CO were in the room, you could die of asphyxiation.
 A naming system that distinguishes between these
 two compounds is needed.
 Writing Formulas for Binary Molecular
             Compounds

How do you write the formula for a binary
  molecular compound?
Use the prefixes in the name to tell you the subscript of
each element in the formula. Then write the correct
symbols for the two elements with the appropriate
subscripts.
 9.3

  Naming Binary Molecular Compounds
Some guidelines for naming binary molecular compounds:
– Name the elements in the order listed in the formula.
– Use prefixes to indicate the number of each kind of
  atom.
– Omit the prefix mono- when the formula contains only
  one atom of the first element in the name.
– The suffix of the name of the second element is -ide.
  Ex. Silicon carbide is a hard
  material like diamond. The
  name silicon carbide has no
  prefixes, so the subscripts of
  silicon and carbon must be
  one. Thus, the formula for
  silicon carbide is SiC.
            9.3 Section Quiz

1. Which of the following compounds is named
  INCORRECTLY?
  a) CS2, carbon disulfide
  b) BCl3, boron trichloride
  c) IF7, iodine heptafluoride
  d) PCl5, phosphorus hexachloride
              9.3 Section Quiz

2. Which of the following molecular compounds is
  named INCORRECTLY?
   a) SbCl3, antimony trichloride
   b) C2O5, dicarbon pentoxide
   c) CF4, carbon tetrafluoride
   d) H3As, hydrogen arsenide
             9.3 Section Quiz

3. The correct formula for tetraphosphorus trisulfide
  is
   a) P3S4
   b) S3P4
   c) P4S3
   d) S4P3
     9.4 Naming and Writing Formulas for
              Acids and Bases
Three rules can help you name an acid with the
  general formula HnX.
1)    When the name of the anion (X) ends in -ide, the acid name begins with
      the prefix hydro-. The stem of the anion has the suffix -ic and is followed
      by the word acid.
2) When the anion name ends in -ite, the acid name is the stem of the anion
   with the suffix -ous, followed by the word acid.
3) When the anion name ends in -ate, the acid name is the stem of the
   anion with the suffix -ic followed by the word acid.
An acid is a compound that contains one or more hydrogen
atoms and produces hydrogen ions (H+) when dissolved in water.
Acids have various uses.


                Summary of Naming Acids
      How are the formulas of acids determined?


Use the rules for writing the
names of acids in reverse to
write the formulas for acids.
   What is the formula for
   hydrobromic acid? Following Rule
   1, hydrobromic acid (hydro- prefix
   and -ic suffix) must be a
   combination of hydrogen ion (H+)
   and bromide ion (Br–). The formula
   of hydrobromic acid is HBr.


     Know these acids (except carbonic) and the other hydro- examples
How are bases named?
Bases are named in the same way as other ionic compounds—the name
of the cation is followed by the name of the anion.
For example, aluminum hydroxide consists of
the aluminum cation (Al3+) and the hydroxide
anion (OH–). The formula for aluminum
hydroxide is Al(OH)3.
Be able to name bases.
          9.4 Section Quiz

1. The name for H2S(aq) is
   a) sulfuric acid.
   b) hydrosulfuric acid.
   c) sulfurous acid.
   d) hydrosulfurous acid.
             9.4 Section Quiz

2. The correct chemical name for NH4OH is
   a) nitrogen tetrahydrogen hydroxide.
   b) nitrogen pentahydrogen oxide.
   c) ammonium oxyhydride.
   d) ammonium hydroxide.
                   Review Questions
•   1. What is a covalent bond?
•   2. List two properties of molecular compounds.
•   3. What is a diatomic element? List the 7 diatomic elements.
•   4. Draw the Lewis structure (electron dot) for O2, H2O, and NH3. Describe
    the bonds and electrons in each.
•   5. Why does the chlorine atom in hydrogen chloride acquire a slightly
    negative charge? What do you call this unequal sharing?
•   6. How are electronegativity values used to determine the probable type
    of bond that will form in a compound?
•   7. Write the prefixes for molecular compounds from 1 to 10.
•   8. Name the molecular compounds CO, CO2, CBr4, ICl, I4O9.
      Ch. 8: What you need to know
8.1
• •Vocabulary: covalent bond, molecule, molecular formula, structural formula
• •Know the difference between ionic and covalent bonds and why elements form each
    bond (structure, melting/boiling point, conductivity, metal/nonmetal)
• •Know a molecular compound has covalent bonds
• •Know at least 5 ways in which to determine whether a compound is ionic or molecular
    (i.e. metal/nonmetal, melting point, electronegativity, drawing, electron dot structures
8.2
• •Vocabulary: single, double, triple covalent bonds
• •Know the 7 diatomic elements and their bonding
• •Be able to draw electron dot structures of simple molecular compounds
8.3 none
8.4
• •Vocabulary: polar and nonpolar covalent bonds, intermolecular force, Van der Waals
    forces, hydrogen bonding
• •Know that water is polar and how to draw a water molecule
• •Know how to use electronegativity values and general bonding to determine type of
    bonds
• •Know relative strength of ionic, covalent, Van der Waals, and hydrogen bonding

								
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