6.3 Chemical Formula and Name of Binary Ionic Compounds

This topic explains how to name binary ionic compounds and how to write their chemical formulas.

Definition

A binary ionic compound is composed of mono-atomic metal cations and mono-atomic non-metal anions.

Simple explanation: A binary ionic compound is made of a metal ion (positive) and a non-metal ion (negative) — just two types of elements.

Main Idea

• The cation is named first.
• The anion is named second.

The name of the cation is the same as the name of the metal, but in the name of a mono-atomic anion, the suffix -ide is added to the root name of the element.

Simple explanation: Metal name first, then non-metal name with -ide at the end.

Core Concept

Naming Binary Ionic Compounds

• Sodium chloride — NaCl
• Magnesium oxide — MgO
• Aluminium nitride — AlN

Detailed Explanation

Writing the Name

To name a binary ionic compound:

• Name the cation first (metal name unchanged).
• Name the anion second (root name + -ide).

Examples:

Writing the Chemical Formula

The following steps are used to write the chemical formula of a binary ionic compound:

Step 1: Write the symbols for the cation first and then the symbol for the anion and their charges.

Step 2: Balance the charges on the cations and anions using the smallest coefficient. The total charge on the cation must equal the total charge on the anion because an ionic compound is neutral.

Step 3: Write coefficient as subscripts for each ion.

Step 4: Write the formula of the ionic compound. For this, leave out all the charges and subscripts which are 1.

Worked Examples (from the Book)

Example 1: Sodium Chloride (NaCl)

Step 1: Write symbols and charges:

Na⁺ Cl^-

Step 2: Balance charges:

1(+1) = (-1)1

Step 3: Write coefficients as subscripts:

Na₁Cl₁

Step 4: Leave out charges and subscript 1:

NaCl

Final formula: NaCl

Example 2: Aluminium Oxide (Al₂O₃)

Step 1: Write symbols and charges:

Al³⁺ O^2-

Step 2: Balance charges:

2(+3) = (-2)3

+6 = -6

Step 3: Write coefficients as subscripts:

Al₂O₃

Step 4: Leave out charges:

Al₂O₃

Final formula: Al₂O₃

Simple explanation: The total positive charge must equal the total negative charge, so you find the simplest whole-number ratio of ions.

Intermediate Level

Why Charges Must Balance

Ionic compounds are electrically neutral as a whole. This means:

Total positive charge = Total negative charge

• 2 Al³⁺ ions = 2 × (+3) = +6
• 3 O²⁻ ions = 3 × (−2) = −6
• Total = 0 → neutral

Simple explanation: The positive and negative charges must cancel out exactly.

Common Cations and Anions

Writing Formulas — More Examples

The Criss-Cross Method

A quick way to write formulas:

• Write the ions with their charges.
• Criss-cross the charge numbers to become subscripts.
• Simplify if needed.

• Al³⁺ and O²⁻
• Criss-cross: Al₂O₃
• (3 goes to O, 2 goes to Al)

• Ca²⁺ and Cl⁻
• Criss-cross: CaCl₂

Simple explanation: The charge on one ion becomes the subscript of the other.

Important Points

• - Binary ionic compound: metal cation + non-metal anion.
• - Naming: cation name first; anion name = root + -ide.
• - Examples: sodium chloride (NaCl), magnesium oxide (MgO), aluminium nitride (AlN).
• - Writing formulas:
• 1. Write cation and anion with charges.
• 2. Balance charges (total positive = total negative).
• 3. Write coefficients as subscripts.
• 4. Leave out charges and subscript 1.
• - Ionic compounds are electrically neutral.
• - Criss-cross method: charge on one ion becomes subscript of the other.
• - Examples:
• - NaCl (1:1)
• - MgO (1:1)
• - Al₂O₃ (2:3)
• - CaCl₂ (1:2)
• - K₂S (2:1)

Remember

• - Name: metal first, non-metal + -ide.
• - Formula: balance charges → total positive = total negative.
• - NaCl: sodium chloride.
• - MgO: magnesium oxide.
• - Al₂O₃: aluminium oxide.
• - CaCl₂: calcium chloride.
• - K₂S: potassium sulfide.
• - Criss-cross the charges to get subscripts.

Common Mistakes

• 1. Forgetting the -ide suffix. Non-metal anions end in -ide (chloride, oxide, sulfide, nitride).
• 2. Not balancing charges. The total positive charge must equal the total negative charge.
• 3. Writing subscripts before balancing. Always balance charges first.
• 4. Forgetting to simplify. Use the smallest whole-number ratio.
• 5. Confusing the criss-cross method. The charge on one ion becomes the subscript of the other ion, not the other way around.
• 6. Writing the anion first. The cation (metal) always comes first in the name and formula.

Quick Revision

• Binary ionic compound: composed of mono-atomic metal cations and mono-atomic non-metal anions.
• Naming: cation named first (metal name unchanged); anion named second (root name + -ide).
• Examples: sodium chloride (NaCl), magnesium oxide (MgO), aluminium nitride (AlN).
• Writing formulas:
• Example: Sodium chloride → Na⁺ Cl⁻ → NaCl
• Example: Aluminium oxide → Al³⁺ O²⁻ → Al₂O₃
• Ionic compounds are electrically neutral.

60-Second Revision

• Binary ionic compound = metal + non-metal.
• Name: metal first, non-metal + -ide.
• Formula: balance charges.
• NaCl = sodium chloride; MgO = magnesium oxide; Al₂O₃ = aluminium oxide.
• Warning: don't forget the -ide suffix and always balance charges.