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:
Each carbon makes 4 bonds and each hydrogen makes 1. Now you can try it yourself for propane.
(republished on 07/13/06)