MCAT Question A Day - 2/13/14 - Answer!

In which of the following does ionic bonding predominate?

I. NH₃

II. CaCl₂

III. PCl₅

IV. K₂O

A. I and III only

B. II and III only

C. I and IV only

D. II and IV only

D. Ionic bonding predominates in those compounds which contain elements with large differences in electronegativity. The larger the difference, the more polar the bond. The easiest, and quickest, way to answer this question is to consult the periodic table. You should know that the most electronegative elements are located in the upper-right hand corner of the table (excluding the noble gases) and those elements that are the least electronegative are located in the lower left-hand corner. Looking at the answer choices, you can see that they are all two-item choices--in other words, look for the two compounds whose elements are the furthest apart in the periodic table. Compound I is ammonia, which you probably should know to be a covalent compound already. Since answer choices A and C both contain roman numeral I, they can be eliminated. Roman numeral II, calcium chloride: calcium is located in the second column, fourth row; chlorine is located in the 17th column, third row--pretty far apart from each other---mostly ionic. You may also know that calcium tends to form the +2 cation, while chlorine forms the –1 anion to complete their octet. Oppositely-charged ions would form ionic bonds. Roman numeral III, phosphorus pentachloride: in the periodic table, phosphorus and chlorine are only separated from each other by sulfur, making it mostly covalent. By elimination, choice D is the correct response.

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