Predict Formula of Ionic Compound

<< Ionic Bonding

 

Given information of the lattice (either diagram or text description), predict the formula of the ionic compound

 

Prerequisite Knowledge

• In an ionic compound, the ions are packed in a regular and alternating pattern.

   

  
• The regular arrangement of ions is called a lattice.
• The formula of an ionic compound is given by the ratio of the number of cations and anions.

 

Given a diagram of a section of the lattice

 

Example:

 

 

Approach:

• Count the total number of cations and anions and obtain the ratio.

 

Total no. of cations (+) = 3

Total no. of anions (–) = 3

Ratio of cations to anions = 1 : 1

 

Formula of compound = XY

 

Given a description of the lattice

 

Example:

 

An ionic compound consists of a lattice of positive ions of metal M and negative ions of non-metal N. Each positive ion is surrounded by eight negative ions and each negative ion is surrounded by four positive ions. What is the formula of the compound?

 

Approach:

• The ratio of the cations to anions is NOT equal to the number of ions surrounding it.

• The total number of cations can be found by multiplying the numbers given in the 2 cases.

• The total number of anions can be found by multiplying the numbers given in the 2 cases.

 

	+	–
	1	8
	4	1
Total	1 x 4 = 4	8 x 1 = 8

 

Ratio of cations to anions = 1 : 2

 

Formula of compound = MN₂