7.1. Ionic Compounds http://www.ck12.org
lsciencespot/uploads/binary%20ionic%20compounds.html?attredirects=0&d=1.
Naming Compounds Using the Stock System
Naming compounds that involve transition metal cations requires the use of the Stock system. Consider the binary
ionic compound FeCl 3. The name "iron chloride" would be ambiguous because iron is capable of forming two ions
with different charges. The name of any iron-containing compound must reflect which type of iron ion is in the
compound. In this case, the subscript in the formula indicates that there are three chloride ions, each with a 1−
charge. Therefore, the charge of the single iron ion must be 3+. The correct name of FeCl 3 is iron(III) chloride,
where the Roman numeral indicates the charge of the cation. Listed below (Table7.4) are a few other examples.
TABLE7.4: Examples of Naming Using the Stock System
Formula Name
Cu 2 O copper(I) oxide
CuO copper(II) oxide
SnO 2 tin(IV) oxideThe first two are both oxides of copper (Figure7.3). The ratio of copper ions to oxide ions determines the name.
Since the oxide ion is O^2 −, the charges of the copper ion must be 1+ in the first formula and 2+ in the second formula.
In the third formula, there is one tin ion for every two oxide ions. This means that the tin must carry a 4+ charge,
making the name tin(IV) oxide.
You can practice naming multivalent metal compounds by completing this online crossword puzzle: https://sites.go
ogle.com/site/pattihowellsciencespot/uploads/multivalent%20metals%20compounds.html?attredirects=0&d=1.
Writing Formulas for Binary Ionic Compounds
If you know the name of a binary ionic compound, you can write its formula. Start by writing the metal ion and its
charge, followed by the nonmetal ion with its charge. Because the overall compound must be electrically neutral,
decide how many ions of each type are needed in order for the positive and negative charges to cancel each other
out. Consider the compound aluminum nitride. The charges on each of these ions can be determined by looking at
the groups in which aluminum and nitrogen are found. The ions are:
Al^3 + N^3 −Since the ions have charges that are equal in magnitude, 1:1 is the lowest ratio of ions that will produce a neutral
compound. As a result, the formula of aluminum nitride is AlN. Another compound, lithium oxide, contains the
following ions:
Li+ O^2 −In this case, two lithium ions are required to balance out the charge of each oxide ion. The formula of lithium oxide
is Li 2 O.
For compounds in which the ratio of ions is not as obvious, an alternative way to determine the correct formula is to
use the crisscross method. In this method, the numerical value of each charge crosses over to become the subscript of
the opposite ion. The signs of the charges are dropped. The crisscross method is demonstrated below for aluminum
oxide.