Which statement best describes the polarity of the molecule Sil_?

A. It is nonpolar, because the bonds between silicon (Si) and iodine

(l) are not polar.

B. It is nonpolar, because the bond polarities cancel each other out in

a tetrahedral molecule.

C. It is polar, because the bond polarities add together in a trigonal-

planar molecule

D. It is polar, because the bond polarities add together in a

tetrahedral molecule.

B. It is nonpolar, because the bond polarities cancel each other out in a tetrahedral molecule.

Explanation:

Since the molecule is incomplete, I will assume it is SI₄, which is silicon tetraiodide.

In this molecule, one atom of silicon, Si, is surrounded by 4 atoms of iodine, I. Hence, in order for the electron densities (bonding orbitals) to be as separate as possible, the iodine atoms will be placed at the vertices of a tetrahedron with the silicon atom in the center.

Si - I bonds are polar, because the electronegativies of Si and I atoms are different. Nevertheless, the tetrahedral geometry of the molecule makes it symmetrical, which will make that the dipoles of the Si - I bonds are canceled.

Hence, the option "B. It is nonpolar, because the bond polarities cancel each other out ina tetrahedral molecule" describes precisely the polarity of the molecule SiI₄.