For the alternative theorists:
- Thread starter paddoboy
- Start date
Fair point, but even if we consider quartz we have a stoichiometric formula, we have a uinit cell formula, and we have an empirical formula. My point was more along the lines of that while yes it might be technically correct to consider a diamond to be a single molecule it's not neccessarily meaningful to do so. Although, in the case of quartz, at least, there are still clearly defined subunits with the different forms of quartz differing in how they are arranged.
Actually, i'm not sure if what I just said makes sense to me...
My favourite fluorite is when the impurities are free electrons.
i understand atoms combine in integral numbers.
it's impossible to have "a molecule" with 7.5 carbons, or any other element for that matter.
But unit cells can have non integral numbers of atoms in them because individual atoms can be shared between more than one unit cell.
for example each table salt unit cell has, as I recall, 1.5 atoms of sodium and 1.5 atoms of chlorine in it.
With regards to the accepted fact re evolution, the following article may have some bearing on the finer details of that evolution.....
http://phys.org/news/2014-06-proof-life-reevaluating-oldest-archean.html
http://phys.org/news/2014-06-proof-life-reevaluating-oldest-archean.html
then the pic you posted isn't the smallest unit.
there MUST be two units to satisfy the equation.
also, what happens to the bonds on the surface of the diamond?
saturated? oxidized?
Nope. Doesn't work that way.
I imagine so, yes.
equally important is the question "How do we accommodate impurities?"
it HAS to.
there is no such thing as "half an atom" in the current context.
either way it goes, there MUST be 2 units and one of them MUST have 7 or 8 atoms (according to your formula).
No it doesn't.
I've already given you a clue.
Yes there can. In a cubic crystal lattice an atom can be shared by up to eight unit cells.
There's your clue (again).
Here's another one: go back and have a look at what I said about table salt.
Nope. You're missing something, and I've given you two clues. Here's a third.
No matter how many cubic unit cells you place together (which is what you do when you multiply):
An atom on a face will always be shared between two cells.
An atom on an edge will always be shared between four cells.
An atom on a corner will always be shared between eight cells.
Very relevant stuff, to be considered next to Trippy's cartoon about the fundies questioning the word "Evolution" with only a couple of letters missing. And this responds to leopold's argument that the fossil record is "incomplete". In short, the article you picked talks about the debate going on over whether certain microscopic structures, thought to be the filaments of very ancient cyanobacteria, are in fact fossils or whether they are "pseudofossils" made instead by geologic processes. In some cases they have been proven to be of inorganic origin. And I think when the fundies caught wind of it, they went on a propaganda spree, hollering "See! The scientists were insisting they had microfossils, but they were proven wrong! Just goes to show science is broken!" (This has been going on for about 20 years).
Another kind of evidence is "biomarkers" -- chemical traces of the existence of life among the oldest of rocks. For example, steranes (derivative or sterols--think cholesterol) have been found in S. African rocks from ca. 2.5 billion years ago. This is at the time cyanobacteria were building our present oxygen atmosphere.
What the fundies fail to recognize (among a jillion other things) is that the history of evolution goes back way, way earlier than the evolution of humans . . . or even animals. Something like 85% of all evolutionary history was in the Pre-Cambrian, when there were only microbes. And most of that time was the era of cyanobacteria, which dominate the pre-Cambrian and are known to be the principal contributors to the oxygenated atmosphere.
Another kind of marker is graphite, known by its Carbon isotope concentrations to be biogenic, since an enzyme in primitive bacteria prefers this isotope. For example, in sedimentary rock from Greenland this type of graphite is seen as striations layered between turbidites (sediment laid by the current). Those rocks are ca. 3.8 billion years old, which makes the creationist focus on the last few percent of evolutionary history all the more irrelevant.
there is no ambiguity about table salt, it's composed of 2 atoms, sodium and chlorine.
there is no "half an atom" anywhere.
there is no ambiguity about table salt, it's composed of 2 atoms, sodium and chlorine.
there is no "half an atom" anywhere.
edit:
i clicked "submit reply" and the forum software said i had to wait 30 seconds between posts
huh?
this is my first post on the board today.
edit2:
i'm leaving both posts.
the forum software never indicated i made the first one.
exchemist
Valued Senior Member
Yes. You two are in fact talking at cross purposes.
The idea of the unit cell in crystallography has nothing to do with molecules, though it is related to empirical formulae. NaCl is the empirical formula of common salt. The unit cell is the repeating pattern of the lattice, just as in your living room curtains. As such, it is merely a geometrical (i.e. mathematical) entity, not a physically distinct object. But of course, if you add up all the portions of the shared atoms in a unit cell, it must come to an integer multiple of the empirical formula.
As to what happens on surfaces and edges of crystals, yes indeed, you have unsatisfied valencies, which makes them reactive. In practice, they react with things from the surroundings, often including oxygen from the air.
No, not really.
In the case of diamond, two types of carbon atoms occupy the corners.
If you look at the 'top' face of the cubic unit cell, two of the corner atoms aren't bound directly to any other carbon in that unit cell, so I was counting half the corners as voids. If you count them as voids you get C[sub]7.5[/sub] if you count them as being occupied you get C[sub]8[/sub].
I was counting them as voids because I was considering a solitary unit cell, and therefore counting them as voids - adjacent unit cells are connected at the faces, not the corners, and so a repeating pattern can be built up using 7.5 carbon atoms because the voids in the corners of one unit cell are occupied by the corner carbon atoms of its neighbouring cells, and vice versa.
$$\frac{4}{1} + \frac {6}{2} + \frac{4}{8} = 7.5$$
In the case of sodium chloride it was late then, as it is now, and I may have miscounted.
Alternatively, you could recognize it as stacked tetrahedra of carbon atoms, with adjacent tetrahedra sharing corners and call it C[sub]5[/sub].
It doesn't matter how big the cube is, half the corners will always be empty because of the way a tetrahedron fits inside a cube.
exchemist
Valued Senior Member
No, not really.
In the case of diamond, two types of carbon atoms occupy the corners.
If you look at the 'top' face of the cubic unit cell, two of the corner atoms aren't bound directly to any other carbon in that unit cell, so I was counting half the corners as voids. If you count them as voids you get C[sub]7.5[/sub] if you count them as being occupied you get C[sub]8[/sub].
I was counting them as voids because I was considering a solitary unit cell, and therefore counting them as voids - adjacent unit cells are connected at the faces, not the corners, and so a repeating pattern can be built up using 7.5 carbon atoms because the voids in the corners of one unit cell are occupied by the corner carbon atoms of its neighbouring cells, and vice versa.
$$\frac{4}{1} + \frac {6}{2} + \frac{4}{8} = 7.5$$
In the case of sodium chloride it was late then, as it is now, and I may have miscounted.
Alternatively, you could recognize it as stacked tetrahedra of carbon atoms, with adjacent tetrahedra sharing corners and call it C[sub]5[/sub].
It doesn't matter how big the cube is, half the corners will always be empty because of the way a tetrahedron fits inside a cube.
OK I see what you are saying. However I would contend that it is a mistake to consider what is bound to what, when describing a unit cell. It seems to me to lead inevitably to difficulties if you mix up the bonding scheme with the geometry of the lattice. So I vote for including the portions of all atoms in the unit cell and for a C8 unit to be thereby defined.
But I will gladly bow to the judgement of any crystallographers present.
this is where i got screwed up.
I spent a good portion of last night sitting here trying to recall whether or not they should be counted >_> It's been far too long since I did any crystallography. Part of what I was saying was that they simply aren't there unless there's an adjacent unit cell.
It really doesn't bother me whether you go C[sub]7.5[/sub] or C[sub]8[/sub], the point I was making in this regard was simply that there is still a smallest repeating unit. I must admit that when I came up with the 7.5 I wondered if I had made an error, but I double checked the calculations, and performed the same calculation with sodium chloride and got a congruent answer - in both cases there was a multiple of 1.5 involved (5*1.5 = 7.5) which is why I kept referring Leopold back to my comments re sodium chloride.
Meanwhile I spent most of yesterday trying to visuallize in my head how to stack 1.5 of anything together in a repeating pattern and failing. It wasn't until I got home lastnight and started looking at the crystalline structure of diamond, again, that I had my 'a-ha!' moment and spotted my potential error - I think the crystal structure I used to do the initial calcs ommitted the missing carbon atoms entirely and so I didn't even realize I wasn't including them in the structure (some structures have regular vacancies so this didn't strike me as being unsual). When I got the same surprising result twice I chalked it up to a 'bromination' moment and accepted it, I didn't even realize I was making a potentially wrong assumption.
I certainly have no problem with using C[sub]8[/sub] instead of C[sub]7.5[/sub] because it only serves to further underscore my original point that there is this smallest repeating unit which could be considered a 'molecule' of diamond.
ok.
the smallest unit of ANY material is either an atom or a molecule.
the smallest unit of table salt is one molecule consisting one each of sodium and chlorine.
now, if you have an unknown material and you get an empirical formula that doesn't resolve to integers, what does this mean?
this is what i based my opinions on.
the smallest unit of ANY material is either an atom or a molecule.
the smallest unit of table salt is one molecule consisting one each of sodium and chlorine.
now, if you have an unknown material and you get an empirical formula that doesn't resolve to integers, what does this mean?
this is what i based my opinions on.
Yes, I understood your point right from the beginning, Leopold.