Question

An lonic compound contains a single metal cation and a single non-metal atom. What's the structure of this molecule? Is this molecule polar or non-polar?
A. Trigonal planar, polar
B. Linear, non-polar
C. Trigonal planar, non-polar
D. Linear, polar

Answer 1

A molecule is polar when there is a disparity in electronegativity among its constitutive atoms and a geometric asymmetry in the distribution of charge density. A chemical bond’s polarity depends on the difference in the electronegativity between the bonded atoms; the greater the difference, the greater the electric dipole moment, and the greater the ionic character of the bond.

Because metals are generally electropositive and nonmetals electronegative, a metal cation and a nonmetal anion would form a very polarized bond. An ionic compound, by definition, consists of atoms held together by a bond of sufficiently significant ionic character. In this question, since you have a single cation and a single anion, there are no nonbonding electrons to distort the otherwise “linear” geometry of the binary electrovalent compound.

This question is rather malformed, however, which is why “linear” is in quotation marks. That is because electrically neutral ionic compounds do not properly exist as discrete entities, but rather as clusters of cations and anions arranged within a crystal lattice. “Molecule” is a term preferentially reserved for chemical species that contain covalent bonds. While molecules themselves can exist as ions, these ions are generally composed of atoms whose intramolecular bonding is covalent.

Likewise, molecular geometry as predicted by VSEPR theory contemplates a central atom bonded to at least two atomic ligands as well as nonbonding electrons, if any. The theory thus cannot be properly applied to a compound composed only of two oppositely charged ions since such a compound has neither a central atom nor nonbonding electrons.

All that said, of the answer choices you’re given, choice D would seem to be the most sensible answer.