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9.5 COMPOSITION OF COMPOUNDS

As you can imagine, there are literally millions of different
chemical compounds from all the possible combinations of over
90 natural elements held together by ionic or covalent bonds.
Each of these compounds has its own name, so there are mil-
lions of names and formulas for all the compounds. In the early
days, compounds were given common names according to how
they were used, where they came from, or some other means
of identifying them. Thus, sodium carbonate was called soda,
and closely associated compounds were called baking soda (so-
dium bicarbonate), washing soda (sodium carbonate), caustic
soda (sodium hydroxide), and the bubbly drink made by react-
ing soda with acid was called soda water, later called soda pop
(Figure 9.12). Potassium carbonate was extracted from charcoal
by soaking in water and came to be called potash. Such com-
mon names are colorful, and some are descriptive, but it was
FIGURE 9.11 The absolute difference in electronegativities impossible to keep up with the names as the number of known
determines the kind of bond formed. compounds grew. So a systematic set of rules was developed to
determine the name and formula of each compound. Once you
know the rules, you can write the formula when you hear the
Figure 9.11 also shows a molecule that has an even charge
name. Conversely, seeing the formula will tell you the system-
distribution. The electron distribution around one atom is just
atic name of the compound. This can be an interesting intellec-
like the charge distribution around the other. This molecule is
tual activity and can also be important when you are reading the
thus a covalent molecule. Thus, a polar bond can be viewed as an
list of ingredients to understand the composition of a product.
intermediate type of bond between a covalent bond and an ionic
There is a different set of systematic rules to be used with
bond. Many gradations are possible between the transition from
ionic compounds and covalent compounds, but there are a few
a purely covalent bond and a purely ionic bond.
rules in common. For example, a compound made of only two
different elements always ends with the suffix -ide. So when you
EXAMPLE 9.5 hear the name of a compound ending with -ide, you automati-
cally know that the compound is made up of only two elements.
Predict if the bonds formed between the following pairs of elements
will be covalent, polar covalent, or ionic: Sodium chloride is an ionic compound made up of sodium and
chlorine ions. Carbon dioxide is a covalent compound with
(a) H and O carbon and oxygen atoms. Thus, the systematic name tells you
(b) C and Br what elements are present in a compound with an -ide ending.
(c) K and Cl

SOLUTION
From the electronegativity values in Figure 9.10, the absolute
differences are
(a) H and O, 1.4
(b) C and Br, 0.3
(c) K and Cl, 2.2
Since an absolute difference of less than 0.5 means covalent, be-
tween 0.5 and 1.7 means polar covalent, and greater than 1.7 means
ionic, then
(a) H and O, polar covalent
(b) C and Br, covalent
(c) K and Cl, ionic

EXAMPLE 9.6
FIGURE 9.12 These substances are made up of sodium and
Predict if the bonds formed between the following pairs of elements will some form of a carbonate ion. All have common names with the
be covalent, polar covalent, or ionic: (a) Ca and O; (b) H and Cl; and term soda for this reason. Soda water (or “soda pop”) was first
(c) C and O. (Answer: (a) ionic; (b) polar covalent; (c) polar covalent.)
made by reacting soda (sodium carbonate) with an acid, so it was
called “soda water.”

240 CHAPTER 9 Chemical Bonds 9-12