Formulas of binary ionic compounds: the crisscross rule

Make the charges cancel

A binary ionic compound has one kind of cation and one kind of anion. Its formula must be electrically neutral: total positive charge equals total negative charge.

Step 1: write the cation with its charge, then the anion with its charge. Table 4.1 gives them: Li⁺, Mg²⁺, Al³⁺, O²⁻, N³⁻, Cl⁻.

Step 2, crisscross: the number of each charge crosses over to become the subscript of the other ion, and the signs are dropped. Li⁺ and O²⁻ give Li₂O.

Check: 2 × (+1) and 1 × (−2) add to zero. Al³⁺ and O²⁻ give Al₂O₃; Mg²⁺ and N³⁻ give Mg₃N₂.

When the charges are equal, as in Ca²⁺ and O²⁻, the crossed subscripts reduce to 1 : 1, so the formula is CaO.

Key terms

Binary compound

Naming compounds like oxides, chlorides, nitrides.

Cation

Written first in the formula.

Anion

Written second in the formula.

Crisscross method

Getting a neutral formula quickly.

Short questions with model answers

1. Q1. Write the formula of aluminium oxide. (p. 54)

   • Al³⁺ O²⁻
   • Al₂O₃
   • 2(+3) + 3(−2) = 0

   Al₂O₃

2. Q2. Write the formula of magnesium nitride. (p. 54)

   • Mg²⁺ N³⁻
   • Mg₃N₂
   • 3(+2) + 2(−3) = 0

   Mg₃N₂

Common mistakes

• ✗ Li⁺ and O²⁻ give LiO₂: the charge stays with its own ion.

  ✓ The charge number crosses to the other ion: Li₂O.

• ✗ Write the signs too: Li₂O¹⁺.

  ✓ Signs are dropped; the compound is neutral.

• ✗ Ca²⁺ and O²⁻ give Ca₂O₂.

  ✓ A formula unit is the smallest ratio, so reduce to CaO.

MCQs

1. 1. Lithium oxide is:

   1. (a) LiO
   2. (b) LiO₂
   3. (c) Li₂O
   4. (d) Li₂O₂
   Show answer

   (c) Li⁺ and O²⁻ crisscross to Li₂O (p. 54).

2. 2. Calcium phosphide is:

   1. (a) CaP
   2. (b) Ca₃P₂
   3. (c) Ca₂P₃
   4. (d) Ca₂P
   Show answer

   (b) Ca²⁺ and P³⁻: 3 Ca and 2 P (MCQ ii, p. 64).

3. 3. Calcium oxide is:

   1. (a) Ca₂O₂
   2. (b) Ca₂O
   3. (c) CaO₂
   4. (d) CaO
   Show answer

   (d) Equal charges reduce to 1 : 1.

4. 4. In the crisscross method, the signs of the charges are:

   1. (a) Dropped
   2. (b) Written as superscripts
   3. (c) Doubled
   4. (d) Added together
   Show answer

   (a) p. 54: signs of the charges are then dropped.

5. 5. Iron(III) chloride, from Fe³⁺ and Cl⁻, is:

   1. (a) FeCl
   2. (b) Fe₃Cl
   3. (c) FeCl₃
   4. (d) FeCl₂
   Show answer

   (c) 1 × (+3) + 3 × (−1) = 0.

Quick revision

• Write the cation and anion with their charges (Table 4.1).
• Cross each charge number to the other ion and drop the signs: Li₂O, Al₂O₃, Mg₃N₂.
• Check the total charge is zero and reduce to the smallest ratio.