Writing Formulas and
Names for Ionic
Compounds
I can write the name and formula for
a binary ionic compound.
A binary ionic compound is a compound made up of two types of
elements ( a metal and a non-metal)
Examples include sodium chloride (NaCl) and Iron (III) oxide (Fe2O3)
Ionic formulas show the ratio of positive ions to negative ions in the
substance.
The ratio must be just right so that the total positive charge will balance the total
negative charge. The sum of the charges must equal zero.
Writing Formulas
1. Look up the charge (oxidation number) of the cation and the anion.
2. If the sum of the charges is zero, write the symbol for the metal followed
by the symbol for the nonmetal.
3. If the sum of the charges is not zero, use a subscript on either or both
elements to balance the charge. Always write the symbol for the metal
first.
Examples
Write the formula for magnesium chloride, aluminum oxide and sodium sulfide
MgCl2, Al2O3, Na2S
Writing Names, when given the formula
1. Write the name of the cation.
2. Write the name of the anion, but drop the standard ending and add
“ide”.
Examples: Write the name of NaF, Ca3N2, K2S
Sodium fluoride, Calcium nitride, potassium sulfide
Writing formulas with transition metals,
given the name.
1. The name will include a roman numeral. The Roman numeral is the
charge on the cation.
2. Look up the charge on the anion.
3. If the sum of the charges is not zero, use subscripts on either or both
elements to balance the charge.
Examples: Write the formula for Copper (II) oxide and Iron(III) Chloride
CuO, FeCl3
Writing names transition metals, when
given the formula.
1. Write the name of the cation (transition metal)
2. You MUST use a Roman numeral that represents the charge in the name
except
A. Zinc, Cadmium- always use 2+
B. Silver- always use 1+
C. Scandium, Yttrium- always use 3+
3. For all other transition metals, use the charge on the anion(s) to determine
the charge of the transition metal. The sum of the charges must equal zero!
4. Write the name of the anion, but drop the standard ending and add “ide”.
Ex: PbCl2,
Lead (II) chloride,
CuO
Copper (II) oxide
Fe2O3
Iron (III) oxide
Writing formulas with POLYATOMIC
IONS
Polyatomic ion- a charged group of atoms that are chemically bonded
together.
OH -1
hydroxide
SO4 -2
sulfate
CO3 -2 carbonate
Use the charge on cation and anion to determine the subscript for each
element or polyatomic ion.
If you need to add a subscript to a
Sodium hydroxide
polyatomic ion, use a parenthesis, ex:
Aluminum sulfate
Potassium carbonate
Magnesium hydroxide- Mg(OH)2
Writing names of compounds that
contain polyatomic ions.
Write the name of the cation, usually the metal.
Write the name of the anion, usually the polyatomic ion.
Exception- Ammonium- NH4 +1 is the only polyatomic cation
Use the chart to find name and formulas for polyatomic ions.
Examples:
(NH4)2SO4
Al(OH)3
Cu(OH)2
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