5.2 Molecular Substances

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5.2 Molecular Substances
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Objectives

• Compare the properties of molecular and ionic
  substances.
• Distinguish among allotropes of an element.
• Apply formulas to name molecular compounds.

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Properties of Molecular Substance

• Molecular substance- a substance that has atoms
  held together by covalent bonds
• Properties are more variable
• Polyethylene plastic soft
• Quartz hard

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Properties

• Low melting and boiling points
• Less soluble in water
• Not electrolytes
• Interparticles forces between molecules are weak
  and easily broken.
• Explains the softness and low melting points of
  most molecular substances

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Properties

• Different enough that the differences can be used
  to classify and separated them from one another.
• Distillation- method of separating substances in
  a mixture by evaporation of liquid and
  subsequence condensation of its vapor.

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Molecular Elements

• Molecules vary greatly in size
• Most elements occur in nature bonded to another
  element, but some occurs as atoms of the same
  element bonded together.
• Molecular elements- atoms of the same element
  bonded together.
• Why? For stability.

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Diatomic Elements

• Seven nonmetal elements are found naturally as
  molecular elements of two identical atoms.
• Diatomic
• Hydrogen H2 - Gas
• Nitrogen N2 - Gas
• Oxygen O2 - Gas
• Fluorine F2 - Gas
• Chlorine Cl2 - Gas
• Bromine Br2 - Liquid
• Iodine I2 - Solid

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Electron Configuration of Diatomic Elements

• Oxygen
• Chlorine
• Nitrogen

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Allotropes

• Allotropes-molecules of a single element that
  differ in crystalline or molecular structure.
• Oxygen and Ozone
• Ozone is the most common form in atomsphere
• Structural differences

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Ozone

• Occurs naturally and is formed from diatomic
  oxygen by lightning or ultraviolet light.
• Sharp odor
• Also formed from TV sets or computer monitors
• Ozone is harmful to living things
• Uses
• Small amounts of ozone is used to purify water
• Treat clothing, carpeting and other material
  damaged by smoke
• Used to remove cigarette smell
• Layer high in our atmosphere helps to shield
  organisms from harmful UV radiation

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Phosphorus Allotropes

• Three allotropes
• White-will ignite spontaneously in air and must
  be stored under water
• Red-used in the strike pad of safety matches
• Black- is a semiconductor
• Each has unique structure and properties
• Figure 5.15 p. 179

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Carbon Allotropes

• Several important ones
• Graphite
• Carbon Blacks
• Diamond
• Charcoals
• Fullerenes
• Linear Acetylenic Carbon

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Graphite

• Most common form of carbon
• Atoms are linked to each other in a continuous
  sheet of hexagons
• Well organized structure
• The looseness between layers is why graphite is
  useful to write with.

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Carbon Blacks

• Make up most of the soot that collects in
  chimneys
• Formed by the incomplete burning of hydrocarbon
  compounds.
• Each microscopic chunk of carbon is make up of
  millions of jumbled chunks of layered carbon
  atoms.
• Lack organization
• Used in the production of printing inks and
  rubber products.

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Diamond

• Hardest natural substance
• Often used on the tips of cutting tools and
  drills.
• Structure every carbon atom is attached to four
  other carbon atoms
• One of the most organized of all substances
• This organization of covalent bonds accounts for
  the hardness of diamonds.
• Formed under extreme pressure and temperature.
• Diamonds range in age from 600 million to 3
  billion years old

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Charcoals

• Poorly organized carbon molecules.
• Produced from the burning of organic matter
• Charcoal is extremely porous ? great surface area
• Useful for removing odors and tastes(activated
  charcoal)

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Fullerenes

• Unusually stable
• Group of highly organized allotropes with
  even-numbered molecular formulas
• The buckminsterfullerene, C60 discovered in 1985
  in soot and the shape confirmed in 1991 (p. 178)
• Some molecules are hollow spheres other are
  hollow tubes with great flexibility.
• After crashing into steel plates at a speed of
  7000 m/s the rebound with their original shape
  intact.
• Potential use as superconductors

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Linear Acetylenic Carbon

• Threadlike allotrope organized into long spirals
  of bonded carbons
• Each spiral contains 300-500 carbon atoms.
• Produced by using a laser to zap a graphite rod
  into a glass container filled with argon gas.
  The allotrope splatters on the glass and is then
  removed.
• Used in microelectronics
• Some linear acetylenic carbons may eventually
  form fullerenes, whereas other form soot.

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Naming Binary Inorganic Compounds

• Organic compounds- compounds that contain carbon
• Inorganic compounds- compounds that do not
  contain carbon
• Molecular Binary Compounds
• Contain only two nonmetal elements
• bonded covalently

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Naming

• Write out the name of the first nonmetal
• Follow it by naming the second nonmetal with its
  ending changed to ide
• You write the first the element that is farther
  to the left on the periodic table, with the
  exceptions of a few compounds that contain
  hydrogen.
• If the elements are in the same group, name first
  the element that is closer to the bottom of the
  periodic table
• A prefix is need when more than one atom is
  present for the first of the two and always used
  for the second atom of a molecule

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Prefixes

• When a vowel combination (o-o or a-o) appears
  next to each other the first of the pair is
  usually omitted.
• Mono- 1
• Di- 2
• Tri- 3
• Tetra- 4
• Penta- 5
• Hexa- 6
• Hepta- 7
• Octa- 8
• Nona- 9
• Deca- 10

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Lets Try It!

• Name the following molecular compounds
• NO
• NO2
• N2O
• N2O5

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Lets Try It!

• Write the formula for the following molecular
  compounds.
• Carbon disulfide
• Carbon monoxide
• Sulfur hexafluoride
• Dinitrogen trioxide

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Common Names

• A few inorganic compounds have common names that
  all scientists use in place of formal names.
• Examples
• Water
• Ammonia
• Also acids and bases

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Table 5. 7 Names of Common Acids and Bases
Formula Name
Acids HCl H2SO4 H3PO4 HNO3 HC2H3O2 Bases NaOH KOH NH3 Hydrochloric acid Sulfuric acid Phosphoric acid Nitric acid Acetic acid (organic compound) Sodium hydroxide Potassium hydroxide ammonia
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Naming Organic Compounds

• The name of even the most complex organic
  compound is based on the name of a hydrocarbon.
• Hydrocarbon- a organic compound that contains
  only the elements hydrogen and carbon.
• Occur naturally in fossil fuels
• Used mainly as fuels and the raw materials for
  making other organic compounds
• Carbon can form four covalent bonds.

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Table 5.8 Hydrocarbons
Formula Name
CH4 C2H6 C3H8 C4H10 C5H12 C6H14 C7H16 C8H18 C9H20 C10H22 Methane Ethane Propane Butane Pentane Hexane Heptane Octane Nonane decane
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Connecting Ideas

• Formulas represent the know composition of real
  substances
• But just because you can write the formula does
  not mean it can exist.

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Review

• Compare ionic and molecular compounds.
• What are the allotropes of carbon and something
  about each?
• What are the allotropes of phosphorous and
  something about each?

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Homework

• Read p. 176-178 and answer questions 1-3
• P. 181 7 8
• P. 183 1-3