Compounds and Molecules by ewghwehws

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									Compounds and
  Molecules
   Section 4.1
         What are compounds?
• Atoms which have combined with other atoms
• They are pure substances
• They can be written using a chemical formula
  – Like H2O, CH4, C12H22O11, etc.
• They have their own physical and chemical
  properties, which may be different than the
  atoms that make them up
• A compound always has the same chemical
  formula
What holds compounds together?
• Chemical bond: The attractive force
  that holds atoms or ions together
• There are numerous types of bonds:
 How strong are chemical bonds?
• Chemical structure: The arrangement of
  bonded atoms or ions within a substance
  – Bond length: average distance between nuclei
    (nucleuses) of two bonded atoms
  – Bond angle: the angle formed by two bonds to
    the same atom
                Models of Compounds
• Models help you “see” the structure of a
  compound by showing how atoms are arranged.




 Structural Formula
                      Ball and Stick model   Space filling model
 model
What are the major types of chemical
            structures?
• Network structures
• Bonded ions
• Molecules
            Network Structures
• Examples:
  – SiO2 (silicon dioxide, quartz, sand)
  – MgF2 (magnesium fluoride)
  – NaCl (table salt)
• All are strong solids
• Lots of energy is needed to break the bonds, so
  all have high melting points
  – SiO2: 1700˚C
  – MgF2: 1261˚C
  – NaCl: 801˚C
                 Bonded Ions
• Some networks are made of bonded ions
• Examples:
  – NaCl (table salt)
  – LiF (lithium fluoride)
  – KI (potassium iodide)
• Oppositely charged ions form bonds that pack
  themselves close together
                   Molecules
• Examples:
  – C12H22O11 (table sugar)
  – H2O (water)
  – H2S (dihydrogen sulfide)
• Much weaker attractions for other molecules
  than in bonded ions or network solids
• Have much lower melting temperatures because
  of that
  – sugar : 186˚C
  – water:    0˚C
  – hydrogen sulfide : -85˚C
   Summary

Chemical Compounds


  Bonded        Network
  Ions          Structures



    Molecules

								
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