6.3 Naming and Writing Ionic Formulas

In the name of an ionic compound made up of two elements, the name of the metal ion, which

is written first, is the same as its element name. The name of the nonmetal ion is obtained by

using the first syllable of its element name followed by ide. In the name of any ionic compound,

a space separates the name of the cation from the name of the anion. Subscripts are not used;

they are understood because of the charge balance of the ions in the compound

Example: Write the name for the ionic compound Mg3N2.

Steps:

STEP 1 Identify the cation and anion. The cation is Mg2+ and the anion is N3-.

STEP 2 Name the cation by its element name. The cation Mg2+ is magnesium.

STEP 3 Name the anion by using the first syllable of its element name

followed by ide. The anion N3- is nitride.

STEP 4 Write the name for the cation first and the name for the anion

second. magnesium nitride

Metals with Variable Charges

We have seen that the charge of an ion of a representative element can be obtained from its group

number. However, we cannot determine the charge of a transition element because it typically

forms two or more positive ions. The transition elements lose electrons, but they are lost from

the highest energy level and sometimes from a lower energy level as well. This is also true for

metals of representative elements in Groups 4A (14) and 5A (15), such as Pb, Sn, and Bi.

In some ionic compounds, iron is in the Fe2+ form, but in other compounds, it has the

Fe3+ form. Copper also forms two different ions, Cu+ and Cu2+. When a metal can form

two or more types of ions, it has variable charge. Then we cannot predict the ionic charge

from the group number.

For metals that form two or more ions, a naming system is used to identify the particular

cation. A Roman numeral that is equal to the ionic charge is placed in parentheses immediately

after the name of the metal. For example, Fe2+ is iron(II), and Fe3+ is iron(III). TABLE 6.6 lists

the ions of some metals that produce more than one ion.

The transition elements form more than one positive ion except for zinc (Zn2+),

cadmium (Cd2+), and silver (Ag+), which form only one ion. Thus, no Roman numerals are

used with zinc, cadmium, and silver when naming their cations in ionic compounds. Metals

in Groups 4A (14) and 5A (15) also form more than one type of positive ion. For example,

lead and tin in Group 4A (14) form cations with charges of 2+ and 4+, and bismuth in

Group 5A (15) forms cations with charges of 3+ and 5+.

Determination of Variable Charge

When you name an ionic compound, you need to determine if the metal is a representative

element or a transition element. If it is a transition element, except for zinc, cadmium, or silver,

you will need to use its ionic charge as a Roman numeral as part of its name. The calculation

of ionic charge depends on the negative charge of the anions in the formula. For example, if we

want to name the ionic compound CuCl2, we use charge balance to determine the charge of the

copper cation. Because there are two chloride ions, each with a 1- charge, the total negative

charge is 2-. To balance this 2- charge, the copper ion must have a charge of 2+, or Cu2+:

CuCl2

Cu?

Cl^-

Cl^-

1(?) + 2(1-) = 0

? = 2+

To indicate the 2+ charge for the copper ion Cu2+, we place the Roman numeral (II)

immediately after copper when naming this compound: copper(II) chloride.

Writing Formulas from the Name of an Ionic Compound

The formula for an ionic compound is written from the first part of the name that describes

the metal ion, including its charge, and the second part of the name that specifies the non-

metal ion. Subscripts are added, as needed, to balance the charge.

Example: Write the correct formula for iron(III) chloride.

STEP 1 Identify the cation and anion.

STEP 2 Balance the charges. The charge of 3+ is balanced by three Cl- ions. 1(3+) + 3(1-) = 0

STEP 3 Write the formula, cation first, using subscripts from the charge

balance. FeCl3