Michael S. answered • 16d
B.S. in Chemistry, Indiana University; Organic Chem Teaching Intern
Short answers first. These use the Pauling electronegativity values printed in most general chemistry texts: K 0.82, C 2.55, S 2.58, N 3.04, Cl 3.16.
A. K-Cl : 3.16 - 0.82 = 2.34 B. N-N : 3.04 - 3.04 = 0.00 C. C-S : 2.58 - 2.55 = 0.03 D. C-Cl : 3.16 - 2.55 = 0.61
An electronegativity difference is always the larger value minus the smaller one, so it is never negative. The sign carries no information; only the size does.
Now the part that the numbers are actually for. A difference is only useful because it predicts what kind of bond you have:
delta EN bond type (the usual textbook guide) -------- ---------------------------------- 0.0 to 0.4 nonpolar covalent 0.4 to 1.7 polar covalent above 1.7 ionic
A. 2.34 is comfortably ionic. Potassium hands the electron over rather than sharing it, and KCl is a white crystalline salt that conducts when molten or dissolved. This is the largest difference of the four by a wide margin.
B. 0.00 exactly, and not because of any rounding. Two atoms of the same element always have the same electronegativity, so any homonuclear bond (N-N, O-O, Cl-Cl, C-C) is exactly zero and perfectly nonpolar. If you ever compute a homonuclear difference and do not get zero, you looked up one of the two atoms wrong.
C. 0.03 is the interesting one, and it is the reason this problem includes it. Carbon and sulfur are different elements, yet the C-S bond is essentially nonpolar. That is why thiols and sulfides behave much more like hydrocarbons than alcohols and ethers do: the C-O bond has a difference of 0.89 and is genuinely polar, while C-S barely is. Sulfur sits below oxygen in the same group but is much larger, so its pull on shared electrons is far weaker.
D. 0.61 is polar covalent. Chlorine gets the partial negative charge and carbon the partial positive. That partial positive on carbon is exactly the site a nucleophile attacks in an alkyl chloride, so this one number is the whole reason substitution reactions of chloroalkanes work.
Three mistakes this problem is built to catch:
1. Subtracting in the order the question names the atoms. Part A written as K minus Cl gives -2.34. Take the absolute value; a bond cannot have a negative polarity.
2. Assuming that two different elements must give a polar bond. Part C is the counterexample. Different element does not mean different electronegativity in any meaningful amount.
3. Using a table that disagrees with your own book. Some texts round to one decimal (K 0.8, C 2.5, S 2.5, Cl 3.0), which gives 2.2, 0.00, 0.00, and 0.5. Every classification above stays the same, so the chemistry does not change, but your grader is marking against one specific table. Use the one in your text and say which you used.
Two things worth knowing that go one step past the question:
The 1.7 cutoff is a rough guide, not a law. HF has a difference of 1.78 and is a molecular gas, not a salt, so it lands on the ionic side of the line while behaving covalently. Treat the boundaries as fuzzy and expect your instructor to avoid borderline cases on an exam.
A polar bond does not make a polar molecule. Carbon tetrachloride has four C-Cl bonds, each with the 0.61 difference you calculated in part D, and the molecule has zero dipole moment because the tetrahedral geometry cancels all four. Bond polarity is step one; molecular shape is step two, and you need both.
