A chemical formula unit is part of a chemical formula. See the example below: 

Watch the following video to 5:12 to remember how to break down a chemical formula unit. 

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How to draw: 

1) Remember that when drawing a ball and stick model you need to use circles to represent each atom

2) Put the chemical symbol (the one or two letter code) for that atom in each symbol. 

3) Connect the atoms in a chemical formula unit together with sticks (bonds). 

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INSTRUCTIONS: 

Draw a model for each of the chemical formula units below: