Chemistry Reference
In-Depth Information
6.2
Naming Ionic Compounds
In most cases, naming ionic compounds involves simply naming both ions. A
huge majority of ionic compounds are made up of one type of cation plus one
type of anion. Thus, to name most ionic compounds, we name the cation first
and then the anion. The more difficult part of the process is learning to name
cations and anions themselves.
The charges on the ions allow us to deduce the formula from the name of
a compound, even though the numbers of each type of ion are not stated in the
name. Writing formulas for ionic compounds requires deducing how many of
each type of ion must be present to have a neutral compound (see Section 5.2).
The number of each type of ion is
not stated explicitly in the name.
Naming Cations
We learned in Chapter 5 that some metals always form monatomic ions having
one given charge in all their compounds. In this topic, we will call this type of
ion the constant type. Other metals form monatomic ions with different charges
(see Figure 5.11). We will call this type the variable type. There are also some
polyatomic cations, but only three of these are important for this course. Thus,
the first step in naming a cation is to decide which of these three types it is:
polyatomic, constant type, or variable type. We name them in different ways.
POLYATOMIC CATIONS
Only three polyatomic cations are important in this course. The
ammonium ion
is very important and the mercury(I) ion is fairly important.
The hydronium ion is important in Chapter 19. Others may be intro-
duced in more advanced texts.
(NH
4
)
(Hg
2
2
)
(H
3
O
)
CONSTANT TYPE CATIONS
Naming the constant type of cation involves naming the element and adding the
word
ion,
unless a compound is being named. For example, is the potas-
sium ion, and is the calcium ion; is potassium chloride. The alkali
metals, the alkaline earth metals, zinc, cadmium, aluminum, and silver are the
most important metals that form ions of the constant type (Figure 6.2). Each of
these metals forms the same ion in any of its compounds, and the charge on the
ion is equal to the classical periodic group number.
K
Ca
2
KCl
VARIABLE TYPE CATIONS
Naming ions of metals that form ions of more than one charge requires distin-
guishing between the possibilities. For example, iron forms and ions.
We cannot call both of these “iron ion” because no one would know which of
the two we meant. For
monatomic
cations of variable type, the
charge
in the
form of a Roman numeral is attached to the element's name to indicate which
ion we are talking about. For example,
Fe
2
Fe
3
Fe
2
Fe
3
is called iron(II) ion and
is
called iron(III) ion. This system of nomenclature is called the
Stock system.
