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Q & A: Making Molecules: Compounds vs. Elements

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Most recent answer: 10/22/2007
Q:
1. How is a molecule og a compound different from a molecule of an element? 2. Write a chemical formula for each of following. (a) a molecule of propane that is made up of three atoms of carbon and eight atoms of hydrogen. (b) a molecule of hydrogen that is made up of two atoms of hydrogen.?
- Slava Orlov Yermak (age 15)
Burnaby Mountain Secondory, BC, Canada, Vancouver
A:
Slava -

The only difference between a molecule of a compound and a molecule of an element is that in a molecule of an element, all the atoms are the same. For example, in a molecule of water (a compound), there is one oxygen atom and two hydrogen atoms. But in a molecule of oxygen (an element), both of the atoms are oxygen.

For your second question, you can look at it like solving a puzzle. Hydrogen can make only one bond to one other atom. Carbon makes four bonds. So if you're looking at a molecule of hydrogen, with two hydrogen atoms, you know that each one will make one bond and that bond will be in between them (H-H). The propane example is a bit more complicated, but I think you can figure it out if you remember that hydrogen makes only one bond and carbon makes four. Here's an example:

Ethane is a molecule with 2 carbon atoms and 6 hydrogen atoms. Since we know that hydrogen can only bond to one other atom, there's no way that there could be a hydrogen in between the two carbons (since it would have to bond to them both). So we know that the two carbons have to be bonded to each other (C-C). But where do the 6 hydrogens go? Well, we know that the carbons will make 4 bonds, and so far they've only got 1 each. This works out nicely, since it means that each carbon can make 3 more bonds. Since there are 2 carbons, that makes 6 bonds which matches our 6 hydrogens:

...H.H...
...|.|...
.H-C-C-H.
...|.|...
...H.H...

Each carbon makes 4 bonds and each hydrogen makes 1. Now you can try it yourself for propane.

-Tamara

(published on 10/22/2007)

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