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Chapter 19 The Representative Elements Groups 1A Through 4A

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Chapter 19 The Representative Elements Groups 1A Through 4A Powered By Docstoc
					       Chapter 19
The Representative Elements:
   Groups 1A Through 4A
    19.1 Survey of the Representative
                Elements
   Elements in group 1A through 8A are called
    representative elements because they display a wide
    range of physical and chemical properties.
   Representative elements display the range of
    possible valence electrons from one in group 1A to
    eight in group 8A.
   The valence electrons of representative elements
    are in s or p orbitals.
   Metals tend to lose their valence electrons to form
    cations with a configuration of the noble gas from
    the preceding period
   Nonmetals tend to gain electrons to form anins with
    a configuration of the noble gas in the same period
Metalloids or semi metals
                  Atomic size and Group anomalies
1. H, Be, B, C, N, O & F have
properties that distinguish
them from their groups due
to their relatively small sizes
2. H is nonmetal and forms
covalent bonds; Li is metal
and forms ionic bonds
3. All oxides of G 2A are ionic
except that of Be (BeO) it is
   2.
covalent also amphoteric.
4. Same is applicable for G 3A
5. G 5A: carbon forms readily C-C
but Si forms readily Si-O
6. Si-Si exists but less stable
the C-C
7. O=C=O exists but O=Si=O is
not stable; Si-O-Si is stable.
8.Si does not form  bonds. Si 3p
valence orbitals do not overlap
with the O- 2p orbitals
    bonding is important for relatively small elements
    of the 2nd period.
   N exists as N≡N due to tendency to form  bonds
   P exists as P4; P large atoms are like Si do not form
    strong  bonds. They prefer to achieve noble gas
    configuration but forming single bonds.
   O (G 6A) exists as O=O; tendency to form 
    bonding
   S does not form  bonding thus it exists as
    S8.
   F has smaller electron affinity than Cl (not expected)
    the small size of F in F-F with 6 lone pairs of
    electrons leads to much greater repulsion compared
    to Cl

Abundance and Preparation (P. 917): Self study
Distribution (Mass Percent) of the 18 Most Abundant
Elements in the Earth's Crust, Oceans, and Atmosphere
Abundance of Elements in the Human Body
              19.2 Group 1A Elements
   ns1 valence electron configuration.
   Comprise H and alkali metals

                  Alkali Metals
Sources and Methods of Preparation of the
           Pure Alkali Metals
      Selected Physical Properties of the Alkali Metals




 Group 1: M(s) + H2O(l)  2M+(aq) + 2 (OH)-(aq) + H2(g)

• Li has the highest ionization potential and the highest oxidation
potential. Due to its small size and large energy of hydration, thus Li+
attracts water effectively and large energy is released when the ion is
formed. Thus formation of Li+ is favored and Li behaves as a strong
•Reducing agent.
Types of Compounds Formed by the Alkali Metals
                with Oxygen
Selected Reactions of the Alkali Metals
             19.3 Hydrogen




Also prepared from the electrolysis of water
                 Covalent hydrides


Hydrides




           Metallic/interstitial hydrides
19.4 Group 2A Elements
             Group 2A Elements
   Ns2 metals. They are called “Alkaline Earth metals”
   Their oxides are basic
    MO(s) + H2O               M(OH)2
   The differences in reactivity among them are
    shown by their reaction with water:
    M(s) + 2H2O               M(OH)2 + H2(g)
     Ca, Ba, Sr react easily with cold water
     Mg reacts with hot water
     Be does not react with water
Selected Physical Properties, Sources, and Methods of
        Preparation for the Group 2A Elements
                   BeCl2 and BeF2
• BeCl2 and BeF2 melts are poor conductors:
  – Therefore they are covalent rather than ionic solids.
Selected Reactions of the Group 2A Elements
         Ions in Natural Waters: Hard Water

   Rainwater is not chemically pure water.
      Contains dissolved atmospheric gases.

      Once on the ground it may pick up a few to about
       1000 ppm of dissolved substances.
      If the water contains Ca2+ and or Mg 2+ ions we say
       that the water is hard.
   Hardness may be permanent or temporary.
                     Temporary Hard Water


   Contains HCO3- ion.
      When heated gives CO32-,
        CO2 and H2O.
      The CO32- reacts with
        multivalent ions to form
        precipitates.
        (for example CaCO3, MgCO3)
   Water softening on a large scale
    is carried out by precipitating
    the multivalent ions using
    slaked lime Ca(OH)2. CaCO3
    would be precipitated
               Permanent Hard Water

   Contains significant concentrations of anions
    other than carbonate.
      For example SO42-, HSO4-.

      Usually soften by precipitating the Ca2+ and
       Mg2+ using sodium carbonate leaving sodium
       salts in solution.
         Water Softening by ion-exchange

   Ion exchange.
     Undesirable
      cations, Mg2+ Ca2+
      and Fe3+ are
      changed for ions
      that are not as
      undesirable, ex.
      Na+.
     Resins or zeolites.
         A typical cation-exchange resin.
when hard water is passed over the cation-exchange
     resin, the ca2+ and mg2+ bind to the resin
              19.5 Group 3A Elements

 ns2np1 elements
   Boron is a nonmetal
    that forms covalent
    bonds; other elements
    are metals
   Al forms significant
    covalent bonding with
    nonmetals; that is why
    Al2O3 has amphoteric
    nature.
Selected Physical Properties, Sources, and Methods of
        Preparation for the Group 3A Elements
Selected Reactions of the Group 3A Elements
                19.6 Group 4A Elements

   They have the
    valence electron
    configuration:
    ns2np2
   Contain the two
    most important
    elements on the
    earth, C and Si
    which form the
    basic of geologic
    world.
   They all form
    covalent bonds
    with nonmetals:
    CH4, SiF4, GeBr4,
    SnCl4, PbCl4 (ِ All
    sp3 hybridization)
Selected Physical Properties, Sources, and Methods of
        Preparation for the Group 4A Elements
Selected Reactions of the Group 4A Elements
        Chapter 20


The Representative Elements:
   Groups 5A Through 8A
             20.1 Group 5A Elements
                 The Nitrogen Family

   ns2np3 valence
    electrons
    configuration
   Nitrogen can
    exist in many
    oxidation states.
   N and P are
    nonmetallic.
   As and Sb are
    metalloid.
   Bi is metallic.
   Bi and Sb tend to be metallic
   But no ionic compounds containing Bi5+ and Sb5+
    are known
   BiF5, SbF5 and SbCl5 are molecular rather than ionic
   G 5A elements can form molecules or ions that
    involve 3, 5 or 6 covalent bonds to the G 5A atom
   NH3, PH3, NF3, and AsCl3. They all behave as Lewis
    base.
   All G 5A elements except N can form molecules
    (MX5) with 5 covalent bonds.
   The ability of G 5A elements to form  bonds
    decreases dramatically after N.
   This is why N exists as N2 molecules; while other
    elements in the group exist as larger aggregates
    containing single bonds: P4, As4, Sb4
The Molecules of
the Types MX3,
MX5, and MX6
Formed by
Group 5A
Elements
The Structures of the
Tetrahedral MX4+ and
Octahedral MX6- Ions
           20.2 The Chemistry of Nitrogen
   Since N2 molecule contains a triple bond, most
    binary compounds (except NH3) containing N
    decompose exothermically to the elements
   In the preparation of NH3 from N2 and H2, too much
    energy is needed to disrupt the N≡N bond.
   Thus, though K (106) is high the reaction is very slow
    at room temperature.
   Haber process is used to prepare NH3 (high
    pressure, high temperature and a catalyst are
    needed)
   Nitrogen fixation: The process of transforming N2 to
    other nitrogen containing compounds
   Nitrogen fixation can be carried out by:
      Haber process (ammonia can be applied to the
       soil as a fertilizer)
      High temperature combustion process in
       automobile engines. NO produced is converted
       into NO2 that with moisture is concerted into NO3-
       that reaches soil.
      Natural. Lightning produces the energy that
       disrupt N2 and O2 molecules producing reactive N
       and O atoms that attack other molecules to form
       nitrogen oxides that convert eventually to NO3-
      Nitrogen-fixing bacteria that reside on the root of
       nodules of plants such as beans and peas. This
       converts N2 to ammonia and other nitrogen
       containing compounds.
   Denitrification: return of N element to the
    atmosphere as N2 gas. Bacteria changes NO3- to N2
The Nitrogen Cycle
Some Common Nitrogen Compounds
          20.3 The Chemistry of Phosphorus
   Chemical properties of P are significantly different
    from N for the following reasons:
      Nitrogen’s stability to form much stronger 
       bonds
      Grater electronegativity of N

      Larger size of P atom

      Availability of empty valence d orbitals on P

   White phosphorus exists as P4: very reactive
    and bursts into flames on contact with air.
       It is commonly stored under water
   Black P and Red P are network solids
(a) The P4 Molecule Found in White Phosphorus
(b) The Crystalline Network Structure of Black Phosphorous
(c) The Chain Structure of Red Phosphorus
    P is essential for plant growth
    Soluble phosphate fertilizer is made by treating
     phosphate rock with sulfuric acid to make
     superphosphate of lime, that is a mixture of :
     CaSO4. 2H2O and Ca(H2PO4)2. H2O
    A reaction of NH3 and P produces NH4H2PO4
    a very efficient fertilizer
                    20.4 The Group 6A Elements
   The valence electron
    configuration is ns2np4
   Non of these elements
    behaves as a metal
   They achive the noble gas
    configuration by adding 2
    electrons to become 2-
    anion
   G 6A elements can form
    covalent bonds with other
    nonmetals
   Due to the presence of
    empty d orbitals (except
    O), they form molecules in
    which central atom is
    surrounded by more than
    8 electrons: SF4 and SF6
                            Group 7A

   ns2 np5 valence electron configuration.
   All nonmetals
   Reactive. Not free in nature. Found as halide (X-) ions.
   Astatine radioactive with t1/2 = 8.3 hrs for its longest living
    isotope
   Very high electronegativities (4, 3, 2.8, 2.5 and 2).
   Ionic bonds with metals and covalent bonds with
    nonmetals in low oxidation states & polar covalent in
    metals in high oxidation states.
Hydrogen Halides
HCl is the most important acid.

HF is used for etching glass.

SiO2(s) + 4HF(aq)  SiF4(g) + 2H2O(l)


         H–X            X  ( g ) H X  (aq)
                                   2O

         Bond energy   Ho                 Ho    Go
    X    (kJ/mol       (kJ/mol)       (J/mol.K)   (kJ/mol)
    F        565         -510             -159       + 102.4
    Cl       427         -366              -96          90
    Br       363         -334              -81          53
     I       295         -291              -64          23
                      Group 8A

   ns2p6 configuration; Un-reactive.
   He. Component of the sun.
     Present in natural gas (from decay of radioactive
      elements).
     Used as Coolant and a rocket pressurizing gas.

   Ne. Used in Luminescent lighting.
   Ar. Used as a non-corrosive atmosphere in
    light bulbs.
   Xe & Kr form compounds with O and F.

				
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Lingjuan Ma Lingjuan Ma MS
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