Each of the following questions consists of two statements, I in the left-hand column and II in the right-hand column. For each question, determine whether statement I is true or false and whether statement II is true or false. Select the appropriate answer combination from the choices below. Choices with a filled CE oval denote that statement II is a correct explanation of the true statement I.
A molecule of silicon tetrachloride,  , is nonpolar |
|
the four bonds in are identical and the molecule has a tetrahedral structure. |
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The correct answer is T, T, CE.
Silicon has four valence electrons. In , the four chlorine atoms are arranged around the central silicon atom. The four bonds are polar, since 
and 
have different electronegativities, but they are identical. The arrangement of electron pairs that best minimizes the repulsion is a tetrahedral arrangement. There are four dipoles in the molecule but, because of the molecule's symmetry, they cancel each other and the molecule is therefore nonpolar. Both statement 
and statement 
are true. Because the bonds are identical and the molecule is tetrahedral, the molecule is nonpolar (i.e., statement is a correct explanation of statement ).