OK, weird, I know. But these are the things I think about when I’m half-asleep in the shower in the morning.
For the sake of example, say we have a chunk of sandstone. If we keep say, cutting it in half, when does it not become sandstone any more? Let’s say it was a quartz and feldspar sandstone. It’s unlikely they’re going to separate away to where you have all of one or the other until you get down to just a few molecules. But before that point, could you still call it sandstone?
Obviously, when we get to the atomic level, if we split a uranium atom, we won’t have uranium anymore, we’ll have the fission products. But at what point, if any, that we had a chunk of uranium and kept dividing it, would it not be uranium any more? Would this be different for uranium since it’s an element rather than a compound like sandstone? Would a homogenous substance like Ivory soap be different?
I realize this is going to be different for almost all substances, and maybe this isn’t GQ material, but I really am looking for a real answer. I also know this is ridiculous, but I figure I might learn something along the way.
I can’t remember much from chemistry classes, but does Avagadro’s number have something to do with this?
(I do realize that this is the basis of homeopathic idiocy too).
If you’re dividing an element (like gold) you’ll need to get down to an atom.
If you’re dividing a collection of molecules (like water) you’ll need to get down to a single molecule.
If you’re dividing an aggregated substance like sandstone, I guess you’d need to go down to a point where the final sample statistically represents the whole. Several times the diameter of the average crystal size maybe?
For an element, the answer is also “one molecule”. Different allotropes of the same element are really different substances, and a single atom of an element is going to behave radically (pun retroactively intended) differently from a molecule of it (except in the case of noble gasses, of course, where a molecule is one atom).
It gets more complicated for crystalline materials (which includes metals), though, since you need some number of fundamental cells to define a crystal (a single one wouldn’t be enough), but once you have enough, you can have any number of fundamental cells. Roughly, I’d say that it’s not a proper crystal until you have at least one fundamental cell for which neither it nor any of its nearest neighbors is at the surface of the crystal, but how many that would take would depend on the crystalline structure (for instance, for a simple cubic crystal, it’d take at least 125 cells).
Elements usually it will be an atom if you can isolate just one. Often only one would be unstable. As for Avagadro’s number, it only relates to man’s use of units. Thus an H2 molecule would have a mass of 2 grams/Avagadro’s number. I don’t know how you would isolate a single molecule of H2 or any reason you couldn’t. I am not sure how you would know that you had it.
(ETA: referring to where Chronos pulled “125 cells” from)
I think he’s saying that the first two “layers” of a crystal have different properties than the bulk material – a somewhat arbitrary choice, but it seems reasonable to me. For a cubic crystal, 125 cells is a 5x5x5 crystal. That’s big enough that there are two layers surrounding the central cell.
It is somewhat arbitrary (if you get right down to it, “edge effects” still apply to some degree any finite distance from an edge), but I was actually thinking more along the lines of “surface layer is different, then at least two layers of non-surface”.