Learning Objective

In this lesson we will learn how ionic formulas reflect the ratio of positive and negative ions in ionic compounds.

Learning Outcomes

By the end of this lesson you will be able to:

  • Write the formula for ionic compounds containing only monatomic ions.
  • Write the formula for ionic compounds containing polyatomic ions.

 
determining the formula for ionic compounds lesson contents

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Lesson Topics

1 | Formulas for Ionic Compounds

2 | Formulas for Compounds Containing only Monatomic Ions

3 | Formulas for Compounds Containing Polyatomic Ions

4 | Exceptions to the Drag-and-Drop Rule

 


Lesson Summary

  • Although ionic compounds are composed of charged particles (ions), they have no overall charge.
  • This is because the total amount of positive charge equals the total amount of negative charge.
  • The formulas for ionic compounds do not represent molecules – they represent the ratio of positive and negative ions in the compound’s lattice structure.
  • The ‘drag-and-drop’ method is a shorthand way of determining the formula for ionic compounds.
  • It involves ‘dragging’ the number in front of each ion and ‘dropping’ it on the bottom right side (subscript) of the other ion.
  • When there is more than one copy of a polyatomic ion in a formula, it needs to have brackets around it.
  • When the size of the charge is the same for each ion in a compound (+1 and –1, +2 and –2, or +3 and –3), the drag-and-drop rule does not apply.
  • The formula of the compound is simply a 1:1 ratio of the two ions.

 
iron iii nitrate ionic formula

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(Header image: Hanna, Adobe Stock)

 


 

forming ionic compounds cut-out activity worksheet   ions and ionic compounds puzzle pieces worksheet   

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Year 9 Chemistry Print Workbook Australian Curriculum

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