hydrocarbons
When non-metal atoms combine to form molecules, they do so by sharing electrons in their outer shells. This type of bonding is called covalent bonding. The number of bonds that an element can form depends on the number of electrons in the outer shell of its atoms.
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Carbon has 4 electrons in its outer shell but really wants 8 to make its outer shell complete, so wants to find other atoms to share 4 electrons with.
Hydrogen only has 1 electron and only needs 1 more to fill its outer shell. Its outer shell is smaller and only needs 2 to make it complete. From this comes hydrocarbons, molecules that are made of combinations of hydrogen and carbon. The simplest hydrocarbon is methane. It is made from one carbon... so how many hydrogens does it electrons share with? This is modelled by the picture on the right. The white balls are hydrogen atoms, the black one is carbon. The yellow sticks represent the covalent bonds. They are at these angles because electron pairs repel each other so that are as far apart as possible. |
What shape does this create? [Hard question: What are the angles made by the yellow sticks?]
The next hydrocarbon is called Ethane. It contains two carbon atoms. Before looking at the pictures below, can you work out what it is going to look like? How many hydrogen atoms will it have?
The next hydrocarbon is called Propane (3 carbons), then Butane (4 carbons). These are also shown in the picture below. What is the pattern in the number of hydrogen atoms? How about the number (yellow) bonds. And the total number of electrons in each molecule?
The next hydrocarbon is called Ethane. It contains two carbon atoms. Before looking at the pictures below, can you work out what it is going to look like? How many hydrogen atoms will it have?
The next hydrocarbon is called Propane (3 carbons), then Butane (4 carbons). These are also shown in the picture below. What is the pattern in the number of hydrogen atoms? How about the number (yellow) bonds. And the total number of electrons in each molecule?
Now you might think is quite simple. But if you think a bit longer you might have asked yourself - is this the only way of arranging these molecules?
Well, no. If you look at the picture below you can see that there are 2 different arrangements for Butane. Can you see why they are different? You might want to consider which other atoms each atom is connected to. Are the number of hydrogen atoms the same?
Well, no. If you look at the picture below you can see that there are 2 different arrangements for Butane. Can you see why they are different? You might want to consider which other atoms each atom is connected to. Are the number of hydrogen atoms the same?
You might think there are more than 2 but remember that if it can be bent to be the same as another arrangement then it is the same, just like when studying trees in graph theory! Hint: you only really need to consider the carbon atoms.
These same-but-different molecules are called isomers, which is related to the definition of two graphs being isomorphic.
It starts to get more complicated when we look at the ways of arranging Pentane (below). How many ways do you think there are of arranging this molecule?
These same-but-different molecules are called isomers, which is related to the definition of two graphs being isomorphic.
It starts to get more complicated when we look at the ways of arranging Pentane (below). How many ways do you think there are of arranging this molecule?
How can you be sure you have found them all? Is there a pattern? Investigate further hydrocarbons such as Hexane and Heptane... Note that this is the same problem as trying to find the number of different trees with n nodes!!