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Alkenes

Before reading through this section it might be a good idea to cover the content of alkanes.

Also check out our video section on Alkanes and Alkenes

Alkenes are unsaturated so they contain double covalent bond between two carbon atoms.  

Ethene

Each carbon atom now shares 2 electrons with another carbon, meaning carbon only has 2 extra electrons to share with 2 other hydrogen atoms. 
Picture
You're almost always likely to come across alkene molecules with just one double bond, although there can be more.

Propene

The next alkene in the series is known as propene, as with the alkanes we can represent the covalent bonds with just lines. This is known as a displayed formula. 
Picture
An alkene means there is a double bond between a pair of carbon atoms, between the other pair there is just a single bond. Each carbon has to have in total of four bonds. The middle carbon has only one bond because it has already three bonds with the other two carbon atoms.
Butene
Picture
Here is the displayed formula of Butene, notice how each carbon has four bonds, one pair of carbons has to have a double bond, this defines an alkene.
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General formula

Just like the alkanes we can use a general formula to work out how many hydrogen atoms there will be on an alkene molecule. 


                                                         CnH2n
Decene

So using the general formula we can work out how many hydrogen atoms there will be on decene (10 carbon atoms)

n=10

2n
2x10 = 20

So Decane has 20 hydrogen atoms 

Picture

Now you should have a better understanding on alkanes and alkenes, attempt the questions below:
hydrocarbon_questions.doc
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Geometric isomerism
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