...
She never had chemistry in school...
Let's see if I can remember this...
An atom contains fo two fieldes, the (what do you call it in english???) center and the shell. The center contains of protons and neutrons. The shell contains of electrons. OK, the protona are positivelly charged (+), and the electrons are negativelly charged (-). The neutrons are neutral (neither + nor -). Pure basics this.
Now, in a substance, let's take litium, since we've allready mentioned it, there is allways a number of protons in the center. As we're talking litium there is allways 3 protons in the center. If there is more or less, it is another substance (if there is two, it's helium, if four, it's beryllium).So, the atomic number a substance is given depends on the number of protons in the center.
Now, over to the atomic weight. Now, as you know there is a WHOLE lot of litium in the universe. So, just talking about one single litiumatom isn't much worth, at least not in these kind of matters. So, lot's of litium in the world, useless to look an one atom.
And, there is difference between litium and litium. We know that the number of protons in the center don't change, but we havn't talked about the NEUTRONS (screw the electrons, theu practiclly don't have any weight anywhay. When it comes to weighting the atoms you only look at the number of protons and electrons). An atom that contains three protons and three neutrons is litium. An atom that contains three protons and four neutrons is allso litium. These are called different isotopes of litium.I don't know how many litium isotopes there are. I would guess on three or four though, and I would allso guess that litium-7 (litium with atomic weight 7, ie 3 protons and 4 neutrons) is the most common
Now let's check what atomic weight really is. It's related to mol. and what is a mol?
Mol is an invented ammount, and one mol is the number of atoms in 1/12 of 12 gram coal-12 (the most common isotope of coal, 6 protons, 6 neutrons). So, one mol coal weights one g. Or something... Anyway, the atomic weight is set so that the average of one mol substance weights it's atomic weight. So, with litium (aw=6,94), the most common one is litium-7. But there is litium-6 and probablly litium-5, so they can't just set the atomic weight to 7. So they check how much litium-7 -6 and -5 there is in the world, count the weight, cound how many mol there is, part weight with mol and comes up with the result 6,94.
Hope you understood ANYTHING of that...
Übereil
