I think I see your point. These chemical equations show the number of each species in a reaction. They may be atoms or molecules.
For many elements an atom is also considered a molecule, but for many gaseous elements molecules have two atoms - which is why we write O
2 rather than 2 O.when elemental Oxygen occurs in an equation.
So you are correct to say
Indranil said:
...
10 molecules of CO, I have 10 atoms of C and 10 atoms of O. Am I right? Yes.
But since oxygen molecules have two atoms, you get only 5 molecules of oxygen.
For the difference between molecules and moles you probably need
to read up on moles (and maybe Avogadro's constant.)
Basically, your equations show the number of atoms and molecules involved in the smallest unit of the reaction. Like an atom of carbon and an atom of oxygen gives a molecule of CO. But this is repeated billions of billions of billions ... of times when you react perceptible amounts of material together.
What we are usually interested in, is the mass or volume of reagents needed or produced. For this it is convenient to talk about moles.
A mole is the relative molecular mass of the substance times 1 g.
Hydrogen has atomic mass 1, molecular mass 2, so a mole of hydrogen is 2 g.
Carbon has atomic mass 12, (commonly*) molecular mass 12, so a mole of carbon is 12 g
Oxygen has atomic mass 16, molecular mass 32, so a mole of oxygen is 32 g
The actual number of atoms or molecules in the mole is not so important as the fact that 1 mole of anything contains the same number of molecules as 1 mole of anything else.
(* molecular weight is not as clear cut as you might hope! But so long as you are consistent, there is no problem.)
Edit: Blow me down! I've just reread Borek's post. This is probably a more accurate representation of the modern mole than my version. But I'd suggest that coming to it through the flawed gram-molecule may be easier to start with?
