ChemPhys, Chapter 20

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Title: ChemPhys, Chapter 20


1
Chapter 20 Chemical Bonds
2
Chemical Compounds
  • The one cent piece is made of copper and is
    copper colored.
  • The Statue of Liberty is covered with copper but
    is not copper colored. Instead it is green.
    Why?
  • The copper on the outside of the statue of
    liberty has reacted with other chemical in the
    atmosphere to make the green compound we see.

3
Compounds Have New Properties
  • When elements combine, the new substances has new
    and different properties.
  • Sodium (Na) is a silvery metal that reacts
    violently with water.
  • Chlorine (Cl) is a poisonous greenish-yellow gas.
  • They combine to make NaCl, table salt.

4
Chemical Formulas
  • A chemical formula tells what elements a compound
    contains and the exact number of atoms of each
    element in a unit of that compound.
  • Water, H2O, has two atoms of hydrogen and one
    atom of oxygen.
  • The subscript (2 in H2O) tells the number of that
    atom in the molecule.
  • No subscript means 1 of that element.

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Familiar Compounds
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Atoms Form Compounds
  • The electric forces between oppositely charged
    electrons and protons hold atoms and molecules
    together. (Positives and negatives attract.)
  • Group 18, the noble gases, generally do not form
    bonds. Why?

7
Atomic Stability
  • The outer energy level of each atom has a place
    for 8 electrons.
  • An atom is chemically stable when its outer
    energy level is complete with 8 electrons.
  • Noble gases have completed outer energy levels
    making them chemically stable and unreactive.

8
Electron Dot Diagrams
  • We can used electron dot diagrams to show the
    outer electrons of an atom.
  • For example

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Energy Levels
  • In the first row, hydrogen and helium have a
    place for only two electrons. So, helium is
    complete with two electrons.
  • From the second row on down, each atom has room
    for 8 electrons. So, the other noble gases are
    complete with 8 electrons.
  • When an atom fills the outer level it is most
    stable.

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When Na and Cl React
  • When sodium and chlorine react, the outer energy
    levels of each are complete
  • Opposite charges hold the atoms together.

Electron Transfer
Na
Cl
Na
Cl-
Table Salt
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When H and O React
  • When hydrogen and oxygen react, the outer energy
    levels of each are complete
  • The atoms share electrons.

H O H
H O H
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Chemical Bond
  • A chemical bond is the force that holds atoms
    together in a compound.
  • This force is the result of the attraction
    between positive and negative charges in atoms
    and molecules.

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Chapter 20, Section 1 Review
  • Describe how a chemical compound differs from its
    component elements.
  • Explain what a chemical formula represents.
  • Explain the electrical forces that exist between
    oppositely charged electrons and protons and how
    that is involved in forming compounds.
  • Why does chemical bonding occur?

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Gain or Loss of Electrons
  • When sodium and chlorine react an electron is
    transferred from Na to Cl
  • An Ion is an atom which has become charged by
    gaining or losing electrons.

Electron Transfer
Na
Cl
Na
Cl-
Table Salt
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Gain or Loss of Electrons
  • When potassium and iodine react an electron is
    transferred from K to I

Electron Transfer
K
I
K
l-
Potassium iodide
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Ionic Bond
  • An ionic bond is the force of attraction between
    the opposite charges of the ions in an ionic
    bond. Now, MgCl2

Mg
Cl
Cl
Becomes
Mg2
Cl-
Cl-
Magnesium Chloride
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Zero Net Charge
  • When an ionic compounds form, the number of
    positive charges equals the negative charges.
  • In NaCl, one positive charge (Na) and one
    negative charge (Cl-)
  • In MgCl2, two positive charges (Mg2) and two
    negative charges (2 x Cl-)

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Sharing Electrons
  • The attraction that forms between atoms when they
    share electrons is known as a covalent bond.
  • A molecule is the neutral particle that forms as
    a result of electron sharing.
  • A single bond is made up of two shared electrons.

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Water is Covalently Bonded
  • The water molecule is held together by two single
    covalent bonds

H O H
Two Single Bonds
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Multiple Bonds
  • A double bond is formed when two atoms share two
    pairs of electrons.
  • A triple bond is formed when two atoms share
    three pairs of electrons

N N ? N N
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Unequal Sharing
  • Sometimes one atom tugs harder on the pair of
    electrons in a covalent bond than the other.
  • In HCl (stomach acid) the chlorine tugs harder on
    the shared pair of electrons than the hydrogen.
  • Since the electrons spend more time on the
    chlorine, it has a partial negative charge.

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Unequal Sharing
  • In HCl the shared pair of electrons spend more
    time on the chlorine than the hydrogen
  • The lowercase Greek letter d is read partial

d-
d
H Cl
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Polar or Nonpolar
  • When two atoms equally share a pair of electrons,
    the covalent bond is nonpolar and there is no
    partial charge on either atom.
  • When two atoms share a pair of electrons
    unequally, then the covalent bond is polar and
    the unequal sharing results in a partial charge
    on each atom.

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Chapter 20, Section 2 Review
  • Describe ionic bonds and covalent bonds.
  • Identify the particles produced by ionic bonding
    and by covalent bonding.
  • What is the difference between a nonpolar
    covalent bond and a polar convalent bond?

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Binary Ionic Compounds
  • A binary compound is one that is composed of two
    elements
  • Examples
  • Sodium chloride NaCl
  • Potassium iodide KI
  • Magnesium chloride MgCl2

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Oxidation Number
  • The number of electrons that an element gains or
    loses is called the oxidation number.

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Oxidation Numbers of Elements
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Special Ions
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Compounds are Neutral
  • When an ionic compounds form, the number of
    positive charges equals the negative charges.
  • In CaF2, two positive charges (Ca2) and two
    negative charges (2 x F-)
  • In Al2O3, six positive charges (2 x Al3) and six
    negative charges (3 x O2-). Six is the lowest
    common multiple of 2 and 3.

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Rules for Writing Formulas
  • Write the symbol of the element or the polyatomic
    ion that has a positive oxidation number.
  • Write the symbol of the element or the polyatomic
    ion that has a negative oxidation number.
  • The charge (without the sign) becomes the
    subscript of the other ion. Reduce the
    subscripts to the ratio.

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Examples of Writing Formulas
  • Lithium Nitrogen ? Li1 N3- ? Li3N
  • Ca O ? Ca2 O2- ? Ca2O2 ? CaO
  • Na Cl ? Na1 Cl1- ? NaCl
  • Iron (III) O ? Fe3 O2- ? Fe2O3

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Writing Names
  • Write the name of the positive ion.
  • Check to see if the positive ion is one of the
    special ones requiring a Roman numeral after it.
  • Write the root name of the negative ion.
  • Drop the ending of the negative element and add
    -ide.

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Names of Ionic Compounds
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Polyatomic Ions
  • Some positive and negative ions have more than
    one atom in them. These are called polyatomic
    ions.
  • Some of the common polyatomic ions are in the
    next table and on charts on the wall of the
    classroom.

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Polyatomic Ions
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Writing Names
  • Write the name of the positive ion first followed
    by the name of the negative ion.
  • K2SO4 becomes Potassium Sulfate
  • Sr(OH)2 becomes Strontium Hydroxide
  • FeCl3 becomes Iron (III) Chloride
  • CuClO3 becomes Copper (I) Chlorate

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Writing Formulas
  • Follow the same rules as with binary compounds.
    Be sure to put polyatomic ions in parentheses
    because they go as a unit.
  • Barium Chlorate Ba2 ClO31-
  • becomes Ba (ClO3)2
  • Ammonium Phosphate NH41 PO43-
  • becomes (NH4)3PO4

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Compounds With Added Water
  • A hydrate is a compound that has water chemically
    attached to each molecule.
  • The word hydrate comes from the word that means
    water.
  • The formula is written followed by a dot and the
    water
  • CoCl2.6H2O (pink) CoCl2 (blue)
  • CaSO4.2H2O

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Naming Binary Covalent Compounds
  • Using what we have learned so far, N2O, NO, NO2,
    and N2O5 would all be nitrogen oxide.
  • To solve this problem, prefixes are added
  • N2O is named dinitrogen oxide
  • NO is named nitrogen oxide
  • NO2 is named nitrogen dioxide
  • N2O5 is named dinitrogen pentoxide (a dropped
    to avoid the ao combination)

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Numerical Prefixes
Number Prefix Number Prefix
1 mono- 6 hexa- 2 di- 7 hepta-
3 tri- 8 octa- 4 tetra- 9 nona-
5 penta- 10 deca-
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Chapter 20, Section 3 Review
  • How do you determine oxidation numbers?
  • How do you write formulas and names for ionic
    compounds?
  • How do you write formulas and names for covalent
    compounds?
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