What elements can be represented by the letter R in organic chemical structures?

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MycelliumMan
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Organic element symbol "R"

I keep hearing different things about what exactly the letter R represents in organic chemical structures. Does the R always represent an alkyl group or can it represent groups with heteroatoms? For example
http://upload.wikimedia.org/wikipedia/commons/thumb/3/3a/Aldehyde2.png/60px-Aldehyde2.png
http://upload.wikimedia.org/wikipedia/commons/thumb/4/48/Ketone-general.png/60px-Ketone-general.png
there's the structure of an aldehyde and a ketone. Can I safely assume that the R's attached of the ketone will be Carbon atoms and will be attached to other C or H atoms. In other words will a ketone always have alkyl groups attached to its O=C or can a ketone have any elements in it?
 
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The 'R' just means an unspecified group/molecule that's attached. You also might see 'X' to denote a halogen (fluorine, chlorine, bromine, iodine, etc.) when it's not important to specify which.

So a aldehyde is a carbon with a double-bonded oxygen and hydrogen on it. What the 'R' stands for there could be anything, as simple as a methyl group, or some crazily complex organic molecule.

Note that the ketone specifically notes it can have two different groups attached to it. So it might have a methyl on one side and an amino on the other side, or an ethyl and propyl, or whatever.
 
MycelliumMan said:
I keep hearing different things about what exactly the letter R represents in organic chemical structures. Does the R always represent an alkyl group or can it represent groups with heteroatoms? For example
http://upload.wikimedia.org/wikipedia/commons/thumb/3/3a/Aldehyde2.png/60px-Aldehyde2.png
http://upload.wikimedia.org/wikipedia/commons/thumb/4/48/Ketone-general.png/60px-Ketone-general.png
there's the structure of an aldehyde and a ketone. Can I safely assume that the R's attached of the ketone will be Carbon atoms and will be attached to other C or H atoms. In other words will a ketone always have alkyl groups attached to its O=C or can a ketone have any elements in it?

The C of the O=C must be bound to others two C atoms, otherwise is not a ketone anylonger: if one is C and the other is H, it's an aldheide (if both are H it's Formaldehyde), if it's OH, is a carboxilic acid, etc.