19.  In each of the following Lewis diagrams, Z represents a main-group element.  Name the group to which Z belongs in each case, and give an example of such a compound or ion that actually exists.
        (a)        ..        ..
                    O=Z=O
                    ^{..        ..}

   The element Z must belong in group IV because it has four electrons that it is sharing with the oxygens in covalent bonds.  An example of such a compound would be CO₂.

        (b)
                   ..          ..
                 :O:      :O:
             ..    I    ..    I    ..
            :O--Z--O--Z--O:
              ..   I    ..    I    ..
                 :O:      :O:
                   ..         ..

The group to which element Z must belong is group VII because it is sharing its seven electrons with the surround oxygen atoms.  Oxygen has six electrons by itself.  An example of this molecule would be Cl₂O₇.

(c)
           ..   ..   ..
        [  O=Z--O: ] ^{-
            ..        ..}

The group to which this element Z must belong is group V because it has five electrons that it is contributing in the bonding process.  An example of this ion would be NO₂^-.

(d)                   ..        _
       [            :O:     ]
        [        ..    I    .. ]
        [  H--O--Z--O:]
        [        ..    I    .. ]
        [            :O:     ]
                       ..

The group that element Z must belong to is group VI because it is sharing six electrons with its neighboring atoms.  An example of this compound is HSO₄^-.

21. Draw Lewis electron-dot formulas for the following species.
        (a)  AsH_{3
                                     ..
                             H--As--H
                                     I
                                    H
       (b)  HOCl}
                      ^{..    ..
                            H--O--Cl:
                                  ..    ..}
       _{(c)  KrF}^-
                                _{..   ..
                            [ :K-Fl: ]} ^{-
                                ..   ..
        (d)}  PO₂Cl₂^{- (central P atom)
                                         ..
                                       :O:
                                  ..     I    ..
                                :Cl--P--Cl:
                                  ..     I    ..
                                       :O:
                                         ..}
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