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Ionic Compound Formula

An ionic compound is a compound made up of positive and negative ions joined together by electrostatic forces of attraction. An ionic compound cannot be assigned a molecular formula since it is not made up of molecules.

The formula of an ionic compound represents the simple whole number ratio of ions in it, that is positive and negative ions.

Ionic Compound Formula is written by considering the following steps.
  1. The formula or the symbol of cation is written first.
  2. It is followed by the formula or symbol of anion. The positive and negative charges are made equal.
  3. In the formula of an ionic compound the ions charges are not shown.
  4. In order to write the ionic compound formula the positive and negative charges on ions can be crossed over to give subscript.
  5. While we write the formula for ionic compound we write the number and not the charges.
Compound is neutral because the total positive charge is equal to the total negative charge.

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Ionic Compound Problems

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Example problems based on the concept ionic compounds formula is given below.

Solved Examples

Question 1: For the following ionic compound write the symbol for the ions present and give the number of each. Li2CO3.
Divide the formula of the compound into the cation and the anion. To accomplish this we have to recognize and remember the composition and charges of common ions.

Li2CO3 is composed of two lithium ions, Ni+ and one carbonate ion CO32-. Li is a Group-1A element and always has a 1+ charge in its compounds. Because the two 1+ charges balance the negative charge of the carbonate ion, the latter must be 2-.


Question 2: Write the formula for the ionic compound of aluminium cation and the fluoride anion.
First we have to decide the formula of the Al cation and the formula of each anion. Combine the Al cation with each type of anion to form an electrically neutral compound.
  • An aluminum cation is predicted to have a charge of 3+ because Al is a metal in Group 3A.
  • Fluorine is a Group 7A element. The charge of the fluoride ion is predicted to be 3-.
  • Therefore we need 3F- ions to combine with one Al3+.
  • The formula of the compound is AlF3.


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