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Predicting the Formulas of Ionic Compounds

Ionic compounds are electrically neutral therefore they have an equal number of positive charges and negative charges. 

Example 1

Predict the formula of Aluminum Oxide

This compound contains aluminum ion and oxygen ion

The Aluminum ion has a charge of +3 and the Oxide ion has a charge of -2 as shown in the periodic table below 

Two aluminum ions would give a total of six positive charges and three oxide ions would give a total of six negative charges making the total charge of the compound to be zero thus electrically neutral.

Then the formula becomes: Al2O3

Tip: In cases like the one above where the two ions have different ionic charges, you can think of it as taking the magnitude of the charges and swapping them to make them into subscripts as shown below to form the compound. 

Example 2

Predict the formula of Sodium chloride

This compound contains sodium ion and chloride ion

The sodium ion has a charge of +1 and the chloride ion has a charge of -1 

One sodium ion gives one positive charge and the chloride gives one negative charge to give a total of zero charges on the compound of sodium chloride. Therefore only one of each ion is needed to form the compound. 

The formula becomes: NaCl

Example 3

Predicting the formula of ionic compounds with polyatomic ions

** Polyatomic ions consist of two or more covalently bonded atoms and it acts as one single ion. 

1)  Predict the formula of Sodium Carbonate

The sodium ion in this compound has a charge of +1 and carbonate is a polyatomic ion with a charge of -2. Two sodium ions (Na+) are needed to give two positive charges and one carbonate ion CO32- )gives two negative charges. They add up to give a total charge of zero, making the compound electrically neutral. 

Therefore the formula becomes: Na₂CO₃

Applying the tip given above gives the following:


2)  Predict the formula of Aluminum Sulfate

The Aluminum ion in this compound has a charge of +3 and Sulfate is a polyatomic ion with a charge of -2. Two Aluminum ions Al3+ ) are needed to give six positive charges and three sulfate ions ( SO42- ) give six negative charges. They add up to give a total charge of zero, making the compound electrically neutral. 

Then the formula becomes: Al2(SO4 )3

Using the above tip given

Polyatomic ions act as one unit therefore you have to put a bracket around them to show they are one unit then put the subscript as shown above

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