Chemistry Reference
In-Depth Information
Lesson 4 - 1 Review
Use the electronegativity values from Figure 4-1a to determine if the
bonds formed in the following substances will be ionic, polar covalent, or
non-polar covalent.
1. CH
4
3. H
2
5. CO
2. NaCl
4. BaI
6. NO
2
Lesson 4 - 2: Ionic vs. Molecular Compounds
Students who are just learning chemistry usually have a great deal of
trouble distinguishing ionic compounds from molecular compounds, which
is understandable, considering how similar their formulas can look. For
example, KCl is an ionic compound, yet HCl is a molecular compound. It is
hard to see and remember this difference, and students often become frus-
trated when they are corrected by their instructor after they incorrectly
refer to an ionic compound as a molecule. How can you quickly tell ionic
compounds from molecular ones without looking up the electronegativi-
ties, so you can avoid the embarrassment of being corrected in class? More
importantly, what is the big deal about the distinction between ionic com-
pounds and molecular compounds anyway?
Recall that ionic compounds are formed from ions, which, in turn, are
formed by gaining or losing electrons. When a large number of these ions
are grouped together in a solid form, we get a crystal that is large enough
to see with the naked eye. Table salt (NaCl) consists of such crystals. What
is both interesting and important to note is that these crystals are not made
up of individual molecules. In fact, there is no special relationship between
any one sodium ion and a neighboring chloride ion. Each sodium ion is
equally attracted to each equidistant chloride ion. There are no special
“partners,” or distinct groups, that make up the ionic crystal.
You see, the formula NaCl only shows us the ratio by which the anions
and cations can be found in the crystal. The fact that there are no sub-
scripts in the formula tells us that for every 1 sodium ion in the crystal,
there is 1 chloride ion. If we have a crystal of CaCl
2
, we don't really have 2
chloride ions attached to or combined with each calcium ion in some type
of special bond. All that formula really tells us is that the ratio of calcium
ions to chloride ions is 1:2.
As you may have guessed from the previous examples, ionic compounds
are also called salts, and they tend to have the following properties.
