[mburt]

Hi guys,

Just a bit of background on me: I completed chemistry in high school and did pretty good, but then when I went to university I didn't take any chem courses for a year. This coming fall I'm going to be enrolled in an introductory university chem course, so I'm doing some prep work now.

The question:

Calculate the total number of atoms in one molecule of trinitrotuluene (TNT), CH₃C₆H₂(NO₂)₃.

My answer:

So in ONE molecule, wouldn't the number of atoms just be the combined number of all its constituents? So is 21 the correct answer?

Thanks,

Mike

[Arkcon]

Yep.  Good work.  You've gotten the terms down straight.  Now, try the rest of the associated problems.

[mburt]

Thanks for your quick response! I'm pretty sure I understand the basic concepts of molecules made up of atoms (and compounds made up of loads of molecules), and ionic compounds which are expressed in ratios of ions, with the convention of formula units (even though the actual pairs of ions may not exist).

It's all starting to come back to me now.

[Dan]

compounds made up of loads of molecules

No, check the definitions again. A molecule is a neutral compound that is held together only by covalent bonds. Compound is just a more general term - a molecule is a sub-type of compound.

[mburt]

I was talking about the actual composition of organic compounds- that they are made up of molecules. Are they not?

[Dan]

No.

http://en.wikipedia.org/wiki/Chemical_compound

and

http://en.wikipedia.org/wiki/Molecule

I drew you a Venn diagram, hopefully that will help clarify it:

[mburt]

Alright I read through the Wikipedia page and the diagram you provided, but I can't help get the feeling that this goes against everything I've learned about compounds in chemistry.

My basic understanding is this (water will be my example):

Hydrogen atoms, H, are found in proportions of 2:1 with oxygen atoms, O, to make a water molecule. I thought that if you had a volume of water, say 1 L, that the 1 L is made up of millions of molecules of H₂0.

So isn't the compound water, made up of water molecules? In my head I just see lots H₂0 interconnected molecules.

I understand that ionic compounds are different altogether - I do have some grasp of it, but maybe that question is for another time - but the question I am asking, I think, is mostly involved with molecules with covalent, and not ionic bonds.

Thanks for your help

[mburt]

Actually, after reviewing your diagram more closely I think I understand now.

I was only considering molecular compounds, which like you said, are both molecules and compounds.

Thanks a lot. I'll save that diagram.

[Dan]

Hydrogen atoms, H, are found in proportions of 2:1 with oxygen atoms, O, to make a water molecule. I thought that if you had a volume of water, say 1 L, that the 1 L is made up of millions of molecules of H₂0.

Yes, absolutely. Millions is an understatement, there are ~3 x 10²⁵ molecules in a litre of water - so somewhere around thirty million million million million!

So isn't the compound water, made up of water molecules? In my head I just see lots H₂0 interconnected molecules.

The fact that there are many of the molecules in the bucket of water is not what makes water a compound - the definition of a compound does not require there to be more than one molecule. I've never seen the term "compound water" used to mean this.

Thanks a lot. I'll save that diagram.

No problem, you're welcome.