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Writing Formulas for Ionic Compounds

Ionic compounds are formed when metals (which lose electrons) combine with nonmetals (which gain electrons). The resulting compound is electrically neutral because the total positive charge equals the total negative charge. Writing the correct chemical formula involves a few systematic steps that ensure charge balance.

1. Identify the Ions

First, determine the cation (positive ion) and the anion (negative ion) involved. For metals, the charge is often indicated by the elements group number, while for nonmetals you can use the typical oxidation states.

Common Cations

ElementCharge
Li+1
Na+1
K+1
Mg+2
Ca+2
Al+3
Fe2++2
Fe3++3

Common Anions

IonCharge
F--1
Cl--1
Br--1
I--1
O2--2
S2--2
N3--3
P3--3

2. Balance the Total Charge

Use the crisscross method:

  1. Write the symbols of the cation and anion.
  2. Write the charge of each ion as a subscript, but do not include the sign.
  3. Cross the charges (the numeric value only) and place them as subscripts on the opposite ion.
  4. If a subscript becomes 1, omit it.
  5. If the same element appears more than once, use parentheses.
Example: Aluminum oxide
Al3+ and O2- crisscross 3 and 2 AlO.

3. Use Polyatomic Ions Correctly

Polyatomic ions behave as a single charged unit. When more than one is needed, enclose the ion in parentheses and indicate the required number with a subscript.

Common Polyatomic Ions

IonFormulaCharge
SulfateSO2
NitrateNO1
PhosphatePO3
CarbonateCO2
AmmoniumNH+1
HydroxideOH1
Example: Magnesium phosphate
Mg2+ and PO3- cross 2 and 3 Mg(PO).

4. Special Cases

  • Transition metals with multiple oxidation states: Use Roman numerals in parentheses after the element name (e.g., iron(III) chloride) or write the proper charge (Fe).
  • Acidic oxides: When an oxide forms a binary acid, the formula is written as HO + oxide HA where A is the nonmetal (e.g., CO HCO).

  • Reference Files For Writing Formulas For Ionic Compounds
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