Gillespie, Chapter 3

Solutions to Homework Problems


6.  Part  Element       Category    Group     Type
      a   Ar, argon     Main Group   VIII     nonmetal
      b   Rb, rubidium  Main Group   I        metal
      c   V, vanadium   Transition   -        metal
      d   Br, bromine   Main Group   VII      nonmetal
      e   Ba, barium    Main Group   II       metal
      f   Fe, iron      Transition   -        metal
      g   Au, gold      Transition   -        metal

13. (a) Li (Group I) has a valence of 1 and S (Group VI) a valence of 2,
        so the product is Li2S

          2 Li(s) + S(s) -----> Li2S(s)      lithium sulfide

    (b) Mg (Group II) has valence 2 and H (Group I) has valence 1,
        so the product is MgH
                             2

          Mg(s) + H (g) -----> MgH (s)       magnesium hydride
                   2              2

    (c) Al (Group III) has valence 3 and Br (Group VII) has valence 1,
        so the product is AlBr3

          2 Al(s) + 3 Br2(l) -----> 2 AlBr3(s) aluminum bromide

    (d) Sr (Group II) has valence 2 and Br has valence 1, so the
        product is SrBr2

          Sr(s) + Br2(l) -----> SrBr2(s)     strontium bromide

15. We need to know the valences for part (a) and (b), which for elements
    in Groups I - IV are the same as the Group number while for elements
    in Groups V - VIII are 8 minus the Group number.

    ELEMENT   Mg   K   Br   P   Si   Al   S   C   F
    GROUP     II   I   VII  V   IV   III  VI  IV VII
    PERIOD     3   4    4   3    3    3   3   2   2
    VALENCE    2   1    1   3    4    3   2   4   1

    (a) Fluorides:   Mg2F  KF  BrF  PF3  SiF4  AlF3  SF2  CF4

    (b) Sulfides:    MgS  K2S  Br2S  P2S3  SiS2  Al2S3  S2  CS2

22. Part  Atom  At.#  Period  Group  Shell      Core
                 (Z)                 Structure  Charge

      a    C      6      2     IV    2,4        +6-2 = +4

      b    Mg    12      3     II    2,8,2      +12-10 = +2

      c    Mg2+  12      3     II    2,8,0      +12-2 = +10

      d    Si    14      3     IV    2,8,4      +14-10=  +4

      e    O      8      2     VI    2,6        +8-2 = +6

      f    O2-    8      2     VI    2,8        +8-2 = +6

      g    S2-   16      3     VI    2,8,8      +16-10 = +6

      h    Br    35      4     VII   2,8,18,7   +35-28 = +7

30. (a) Ca (Group II) loses 2 e- to give Ca2+ while I (Group VII) gains
        one e- to become I-

                                       .
     .Ca. -----> Ca2+  + 2 e-    and   :I: + e- -----> I-
                                       ¨
                                                  ..
     then   Ca2+ + 2 I- -----> CaI2   ionic  Ca2+[:I:-]2
                                                  ¨
                                        .
     (b) .Ca. -----> Ca2+ + 2 e-   and  .O: + 2 e- -----> O2-
                                        ¨
                                                ..
     then  Ca2+ + O2- -----> CaO    ionic   Ca2+:O:2-
                                                ¨
            .                            .
     (c)  .Al. -----> Al3+ + 3 e-   and  .S: + 2 e- -----> S2-
                                         ¨
                                                        ..
     then  2 Al3+ + 3 S2- -----> Al2S3   ionic   [Al3+]2[:S:2-]3
                                                        ¨
                                        .
     (d) .Ca. -----> Ca2+ + 2 e-  and  :Br: + e- -----> Br-
                                        ¨
                                                   ..
     then  Ca2+ + 2 Br- -----> CaBr2   ionic  Ca2+[:Br:-]2
                                                   ¨
                                     .
     (e)  Rb. -----> Rb+ + e-  and  :Se. + 2 e- -----> Se2-
                                     ¨
                                                    ..
     then  2 Rb+ + S2-  -----> Rb2S    ionic  [Rb+]2:S:2-
                                                    ¨

33. (a) A single covalent bond consists of a pair of electrons shared 
        between two atoms; the mutual attraction of each atom for the same
        electron pair bonds them together, 
                                      .. ..      .. ..
              as in the F2 molecule   :F:F:  or  :F-F:
                                       ¨ ¨        ¨ ¨

    (b) A double covalent bond consists of two pairs of electrons shared
        between two atoms; the mutual attraction of each atom for the same
        electron pair bonds them together,
                                       ..  ..      .. ..
              as in the O2 molecule    :O::O:  or  :O=O:
                                        ¨  ¨        ¨ ¨

    (c) A TRIPLE covalent bond consists of three pairs of electrons shared
        between two atoms; the mutual attraction of each atom for the same
        electron pair bonds them together,

              as in the N2 molecule   :N:::N:  or  :N{3 horizontals}N:
                                                           _
              Yes, I agree, we need more symbols because :N=N: looks not
              like a triple bond by rather a double bond with a negative
              charge...not at all intended.  :N#N: ?

    (d) A shared (bonding) electron pair is a pair of electrons shared
        between two atoms that bonds them together; e.g., any of the 
        electrons shared in bonding of F2, O2, or N2.

    (e) An UNshared (nonbonding) electron pair is any pair of electrons
        in the valence shell of an atom that does NOT participate in
        chemical bonding, that is, is not shared with another atom; for
        example, any of the three electron pairs associated with each
        F atom in the F2 molecule that don't partipate in the F-F bond.

38. (a) The ELECTRONEGATIVITY of an atom is its relative power of attraction
        for the electron pair OF A COVALENT BOND.  The attraction results
        from the electrostatic attraction of the nucleus of an atom for a
        pair of electrons in its valence shell, which is proportional to
        the magnitudes of the core charge and the electron charge, and
        inversely proportional to the square of their distance apart
        Coulomb's Law,  electrostatic force proportional to  Q1Q2/r²

        So the electronegativity of an atom will depend primarily on
        (i) its CORE CHARGE, and (ii) the distance of the valence shell
        electrons from the nucleus.

    (b) CORE CHARGE increases ACROSS ANY PERIOD from left to right from
        +1 (Group I) to +8 (Group VIII), and the valence electrons are in
        the same shell, n, so their distances from the nucleus are about
        the same.  Thus, electronegatity is expected to increase from
        left to right across any period, as is observed.  In ANY GROUP,
        the core charge remains the same (equal to the group number) but
        the distance of the valence shell fro the nucleus progressively
        increases as the number of filled inner shells of electrons
        increases.  Thus, the Coulombic force of attraction for the outer
        valence electrons decreases, and electronegativity is expected to
        decrease, in descending any Group, as is observed.

41. Compound                 Bond Type       Explanation

    (c) O2                   Covalent        bond between IDENTICAL atoms

    (d) SO2 (e) ClO2 (f) NO  Polar Covalent  bonds between NONMETALS
                                             of different electronegativities

    (a) Li2O (b) MgO         Ionic           METAL-NONMETAL bonds

50.     Molecule  Lewis Structure        Type    Shape

                     ..
    (a)  H2S      H - S - H              AX2E2   bent
                      ¨
                 ..  ..   ..
    (b)  NF3     :F - N - F:             AX3E1   triangular pyramidal
                  ¨    \  ¨ 
                       :F:
                        ¨
    (c)  BeH2    H - Be - H              AX2      linear

                        H
                       /
    (d)  GeH4     H - Ge - H             AX4      tetrahedral
                       \
                        H

51. (a) Four single bond domains each occupy 1/4 of the volume in the
        valence shell of a central atom to give a TETRAHEDRAL arrangement
        of electron pair domains.

    (b) Three single bond domains and a lone pair domain also adopt
        a TETRAHEDRAL arrangement.

    (c) Two single bond domains and two lone pair domains do likewise.

    (d) One single bond domain and three lone pair domains do likewise.

    (e) One double and two single bond domains arrange themselves in the
        (AX3) TRIANGULAR PLANAR bonding but the double domain is larger
        than either single bond domain.

    (f) Two double bond domains each take up half of the volume of the
        valence shell of the central atom and have a LINEAR arrangement.

    (g) A single bond domain and a triple bond domain are also LINEAR.

    (h) A triple bond domain and a lone electron pair are also LINEAR.

53. Molecule  Lewis Structure  Type  Shape
               ..        ..
    BeCl2     :Cl - Be - Cl:   AX2   Linear  Cl - Be - Cl
               ¨          ¨
                    ..
                   :Cl:                                   Cl
                    |                                     |
    BCl3            B          AX3   Planar triangular    B
                  /   \                                 /   \
               :Cl:   :Cl:                            Cl     Cl
                ¨       ¨
                    ..
                   :Cl:                              Cl     Cl
               ..   |   ..                             \   /
    CCl4      :Cl - C - Cl:    AX4   Tetrahedral         C
               ¨    |    ¨                             /   \
                   :Cl:                              Cl      Cl
                    ¨
               ..   ..  ..                                    N
    NCl3      :Cl - N - Cl:    AX3E1 Pyramidal triangular    / | \
               ¨     \   ¨                                 Cl  Cl Cl
                     :Cl:
                      ¨
               ..   ..  ..                               O
    OCl2      :Cl - O - Cl:    AX2E2 Bent (angular)     /   \
               ¨    ¨    ¨                            Cl     Cl
                 ..   ..
    FCl         :Cl - F:       AX1E3  Linear    F - Cl
                 ¨    ¨

Chris Parr University of Texas at Dallas 2601 North Floyd Road (for deliveries) P.O. Box 830688 M/S BE2.6 (for snailmail) Richardson, TX 75083-0688
Voice: (214) 883-2485 Fax: (214) 883-2925 BBS: (214) 883-2168 (HST) or -2932 (V.32bis) Internet: parr@utdallas.edu (sends Chris e-mail.)

Last modified 12 June 1996.