Fall 2011 Midterm Exam Q6B(c)

[904103354]

The question asks which compound is more ionic. Between LiH and HCl, LiH is considered more ionic. I looked this up online and found a Yahoo answer that says:

Lithium hydride is a network solid and thus is assumed to be "ionic" despite the low electronegativity difference. Based on difference in electronegativity, a lithium-hydrogen bond has about 30% ionic character while HCl has about 20% ionic character. HCl does not form a network structure, but exists as discrete molecules. The fact that LiH is a network solid suggests to some that it is more ionic, even though many compounds with clearly covalent bonds also exist as networks.

This kind of makes sense to me, but I'm looking for an answer/rationalization of why LiH is more ionic that relates directly to how the textbook and lectures present this material. Why is LiH considered more ionic than HCl? If you literally went and calculated EN difference, the EN difference is greater between Li and H than H and Cl, but you wouldn't be able to do this on an exam. How could you reason out why LiH is more ionic than HCl sans electronegativity chart?

[Chem_Mod]

Using the electronegativity values LiH has a difference of 1.1 and HCl has a difference of 0.9. This is pretty close and it is pretty tough to memorize those values. However a Lewis structure for LiH shows two electrons shared between Li and H. One thing you should memorize is that H is more electronegative than what it appears based on its location on the periodic table (2.1 value) whereas Li is 1.0. Thus we will get H^- which is a very reactive species called hydride. Hydride has ionic character since it has a single electron on a small H atom. Li is likewise very electropositive since it it has a low electronegativity value. These two factors coupled together make LiH more ionic then HCl.

[704320688]

Can't you think of Lithium and Hydrogen as an actual ionic bond (metal + nonmetal) so it fits the definition and hydrogen and Chlorine (nonmetal + nonmetal) so no matter the differences, an actual ionic bond beats any trend on the periodic table?