Writing Formulas and Names for Ionic Compounds

How to Write Ionic Formulas

Ionic compounds are formed when a metal (which forms a positive ion, or cation) transfers electrons to a nonmetal (which forms a negative ion, or anion). The compound formed is neutral, which means the total positive charge must equal the total negative charge.

We can use the "Criss-Cross Method" to easily find the chemical formula.

Example: Aluminum Oxide

1. Write the symbols and charges for the ions.

• Aluminum is Al³⁺

• Oxygen (as an oxide ion) is O²⁻

2. "Criss-cross" the numbers. The number from the charge of one ion becomes the subscript for the other ion. Ignore the +/- signs.

• The 3 from Al³⁺ moves to O.

• The 2 from O²⁻ moves to Al.

3. Write the final formula.

• Al₂O₃

Important Note: If the subscripts can be simplified, you must reduce them to the lowest whole-number ratio. For example, for Calcium Oxide (Ca²⁺ and O²⁻), the criss-cross method gives Ca₂O₂, which simplifies to CaO.

Practice Problems

Write the correct chemical formula for each of the following ionic compounds.

Part 1: Simple Binary Compounds

1. Sodium Chloride: ___________________

2. Magnesium Sulfide: ___________________

3. Aluminum Bromide: ___________________

4. Potassium Oxide: ___________________

5. Calcium Nitride: ___________________

Part 2: Compounds with Polyatomic Ions

(Remember to use parentheses if you need more than one polyatomic ion!)

6. Sodium Nitrate: ___________________

7. Calcium Hydroxide: ___________________

8. Aluminum Sulfate: ___________________

9. Magnesium Phosphate: ___________________

10. Ammonium Chloride: ___________________

How to Name Ionic Compounds

Naming ionic compounds follows a set of rules. The key is to identify the type of ions involved: simple metal and nonmetal, a metal with multiple possible charges, or a polyatomic ion.

Rule 1: Simple Binary Compounds (Metal + Nonmetal)

1. Write the name of the metal cation first.

2. Write the base name of the nonmetal anion and change the ending to -ide.

• Example: NaCl

• Na = Sodium

• Cl = Chlorine → Chloride

• Name: Sodium Chloride

Rule 2: Compounds with Transition Metals

Transition metals (and some others, like lead and tin) can form more than one type of positive ion. We use a Roman numeral in parentheses to show the charge of the metal cation.

1. Name the metal cation.

2. Determine the charge on the metal by looking at the anion's charge. The total charge of the compound must be zero.

3. Write the charge as a Roman numeral in parentheses.

4. Name the anion with the -ide ending.

• Example: FeCl₃

• We know each Cl is Cl⁻. There are three of them, for a total negative charge of 3-.

• To balance this, the one Fe ion must be Fe³⁺.

• Name: Iron (III) Chloride

Rule 3: Compounds with Polyatomic Ions

1. Name the cation (metal or polyatomic cation like NH₄⁺).

2. Name the polyatomic anion exactly as it is written in the chart.

• Example: Ca(NO₃)₂

• Ca = Calcium

• NO₃⁻ = Nitrate

• Name: Calcium Nitrate

Practice Problems

Write the correct name for each of the following ionic compounds.

Part 1: Simple Binary Compounds

1. K₂O: ___________________

2. AlBr₃: ___________________

3. MgS: ___________________

4. Ca₃P₂: ___________________

Part 2: Compounds with Transition Metals

5. CuO: ___________________

6. FeCl₂: ___________________

7. PbS₂: ___________________

Part 3: Compounds with Polyatomic Ions

8. KNO₃: ___________________

9. Al(OH)₃: ___________________

10. (NH₄)₂SO₄: ___________________

Writing Formulas for Ionic Compounds - ANSWER KEY

Part 1: Simple Binary Compounds

1. Sodium Chloride:
   Ions: Na⁺ and Cl⁻
   Criss-cross: Na₁Cl₁
   Formula: NaCl

2. Magnesium Sulfide:
   Ions: Mg²⁺ and S²⁻
   Criss-cross: Mg₂S₂ (Simplifies to 1:1 ratio)
   Formula: MgS

3. Aluminum Bromide:
   Ions: Al³⁺ and Br⁻
   Criss-cross: Al₁Br₃
   Formula: AlBr₃

4. Potassium Oxide:
   Ions: K⁺ and O²⁻
   Criss-cross: K₂O₁
   Formula: K₂O

5. Calcium Nitride:
   Ions: Ca²⁺ and N³⁻
   Criss-cross: Ca₃N₂
   Formula: Ca₃N₂

Part 2: Compounds with Polyatomic Ions

6. Sodium Nitrate:
   Ions: Na⁺ and NO₃⁻
   Criss-cross: Na₁(NO₃)₁
   Formula: NaNO₃

7. Calcium Hydroxide:
   Ions: Ca²⁺ and OH⁻
   Criss-cross: Ca₁(OH)₂ (Parentheses are needed)
   Formula: Ca(OH)₂

8. Aluminum Sulfate:
   Ions: Al³⁺ and SO₄²⁻
   Criss-cross: Al₂(SO₄)₃ (Parentheses are needed)
   Formula: Al₂(SO₄)₃

9. Magnesium Phosphate:
   Ions: Mg²⁺ and PO₄³⁻
   Criss-cross: Mg₃(PO₄)₂ (Parentheses are needed)
   Formula: Mg₃(PO₄)₂

10. Ammonium Chloride:
    Ions: NH₄⁺ and Cl⁻
    Criss-cross: (NH₄)₁Cl₁
    Formula: NH₄Cl

Naming Ionic Compounds - ANSWER KEY

Part 1: Simple Binary Compounds

1. K₂O:
   K = Potassium, O = Oxide
   Name: Potassium Oxide

2. AlBr₃:
   Al = Aluminum, Br = Bromide
   Name: Aluminum Bromide

3. MgS:
   Mg = Magnesium, S = Sulfide
   Name: Magnesium Sulfide

4. Ca₃P₂:
   Ca = Calcium, P = Phosphide
   Name: Calcium Phosphide

Part 2: Compounds with Transition Metals

5. CuO:
   Oxygen is O²⁻, so the one copper ion must be Cu²⁺.
   Name: Copper (II) Oxide

6. FeCl₂:
   Each chlorine is Cl⁻, so two make a 2- charge. The one iron ion must be Fe²⁺.
   Name: Iron (II) Chloride

7. PbS₂:
   Each sulfide is S²⁻, so two make a 4- charge. The one lead ion must be Pb⁴⁺.
   Name: Lead (IV) Sulfide

Part 3: Compounds with Polyatomic Ions

8. KNO₃:
   K = Potassium, NO₃ = Nitrate
   Name: Potassium Nitrate

9. Al(OH)₃:
   Al = Aluminum, OH = Hydroxide
   Name: Aluminum Hydroxide

10. (NH₄)₂SO₄:
    NH₄ = Ammonium, SO₄ = Sulfate
    Name: Ammonium Sulfate