# The number of constituent particles in one mole of a given substance.June 8, 2013 2:52 AM   Subscribe

Avogadro Project - The International Avogadro project relates the kilogram to the mass of a fixed number of atoms by measuring the number of atoms in a sphere of silicon. I'll leave this here.
posted by hypersloth (26 comments total) 5 users marked this as a favorite

I hope they're going to be exact. No rounding to the nearest billionth!
posted by blue_beetle at 3:41 AM on June 8, 2013

And I'll just leave this here.
posted by infini at 3:45 AM on June 8, 2013

Good stuff. I'm amazed that they're letting people handle their unit of measure like that. I admit I don't have a model in my head on how easy it is to cause an object to shed an atom, but even little effects like minute static charges could cause atomic bombardment and either chip an atom off or else embed an atom under the surface.

Of course, that could just be an early prototype that they can show to journalists so that if someone were to drop it, they wouldn't be knocked back a year or two.
posted by YAMWAK at 5:07 AM on June 8, 2013

Well, they'll have done their measurements - the sphere will change over time anyway, and the point is rather to remove the need to have a prototype weight anyway. So I reckon that's going to be more of a ceremonial orb from hereon in.

I think the most fascinating aspect was how the 40 reference weights diverged in mass over time. That's an unexpected result, and I wonder what theories are being put forward to explain it.
posted by Devonian at 5:21 AM on June 8, 2013 [1 favorite]

Begin by choosing avogadro's constant to be an integer. These folks don't like the idea of measuring a sphere as Using a sphere precludes choosing an integer at all, because of the irrationality of p. On the other hand, if the measurement process goes better with a sphere, I'm all for that. (of course, there are still an integral number of atoms in a sphere of just like there are in a cube)
posted by jepler at 6:20 AM on June 8, 2013

The implications of this are pretty heavy.
posted by Nanukthedog at 6:45 AM on June 8, 2013

It's not just the measurement process, but the manufacturing process. A sphere is more symmetric than a cube (or any other shape), and that spherical symmetry makes your life a lot easier. As a trivial example, imagine trying to measure the width of a cube so you can grind it to some precise size: how can you verify that the side opposite your measurement probe / grinding surface is perfectly parallel with the side you're grinding? Etc. Spheres are a PITA to work with, but are inherently simpler in the most important ways (for this project) because of that symmetry.
posted by introp at 6:50 AM on June 8, 2013 [3 favorites]

The implications of this are pretty heavy.

If not massive.
posted by peeedro at 6:58 AM on June 8, 2013

When I heard about the standard unit of the kilogram shedding/gaining weight or whatever it is doing to make scientists angry, I heard about another effort to define kilogram by some atomic vibrational method or somesuch.

This silicon sphere effort seems somewhat akin to the original method. Why do they have to make a model at all? If they know the mass of one atom, can't they just agree on how many atoms of whatever element they chose = a kilogram?
posted by pashdown at 7:13 AM on June 8, 2013

If not massive.

True. I'll be poundering your words the rest of the afternoon.
posted by Nanukthedog at 7:27 AM on June 8, 2013

Dude! WE HAVE TO STEAL IT!
posted by straight at 7:31 AM on June 8, 2013 [3 favorites]

pashdown, the wikipedia kilogram article has two other proposals for redefining the SI kilogram: the watt balance and ampere-based force. I don't know that either of these is the one you're thinking of, as "atomic vibration". You could be thinking of the redefinition of the SI second in terms of atomic vibrations in 1967. (Formerly, the second was defined in terms of a fraction of either the day or year, a definition which is unsatisfactory if only because it changes over time but also because it did not have the desired precision)
posted by jepler at 8:07 AM on June 8, 2013

I hope they're going to be exact. No rounding to the nearest billionth!
The plan is to create a new 'standard' to measure things by. Rather then say that a kilogram is the weight of a particular object in France, you will be able to create your own standard to calibrate scales by by creating a sphere of pure silicon with a certain diameter. It only needs to be as accurate as the resolution of your scale, though.
This silicon sphere effort seems somewhat akin to the original method. Why do they have to make a model at all? If they know the mass of one atom
It is based on the mass of one atom, specifically the number of atoms it would take to make a sphere of diameter X. The reason they need to make the model, though is both to prove it can be done and also because you need something to actually calibrate scales too
posted by delmoi at 8:07 AM on June 8, 2013

(all the good reasons for defining the SI second as an integral number of countable events apply to defining Avogadro's constant as an integral number of countable atoms)
posted by jepler at 8:08 AM on June 8, 2013

Good stuff. I'm amazed that they're letting people handle their unit of measure like that. I admit I don't have a model in my head on how easy it is to cause an object to shed an atom, but even little effects like minute static charges could cause atomic bombardment and either chip an atom off or else embed an atom under the surface.
Well the whole idea is that there's nothing special about that particular sphere except for the fact it cost a shitload of money to create. You could create another sphere using the same materials and methods and it would have the same weight (down to your measuring resolution), and therefore scales across the universe can be calibrated with each-other without needing to share the same calibration targets.
posted by delmoi at 8:13 AM on June 8, 2013 [1 favorite]

As I understand it the purpose of the sphere is not to create a more accurate mass (directly) but to get a more accurate quantity for Avagadro's number by calculating the number of silicon atoms in the sphere. After that they know how many Carbon-12 atoms are in 12 grams and thereby have a definition of a kg in terms of C12 atoms.
posted by Mei's lost sandal at 8:27 AM on June 8, 2013 [1 favorite]

Shouldn't it be in the shape of a silicon crystal for absolute precision?
posted by humanfont at 9:38 AM on June 8, 2013

As I understand it the purpose of the sphere is not to create a more accurate mass (directly) but to get a more accurate quantity for Avagadro's number by calculating the number of silicon atoms in the sphere.

Wouldn't a perfect atomic sphere be easier to do in a simulation than IRL?
posted by pashdown at 11:50 AM on June 8, 2013 [1 favorite]

Why are they using silicon? It is only the second most pure crystal they can make: High purity germanium would be better, wouldn't it?
posted by Canageek at 12:10 PM on June 8, 2013

Since molecules are more or less spheres, it's probably easier to just extrapolate the sphere up than it is to measure all the roundy curves on the edges of a non spherical object.

Begin by choosing avogadro's constant to be an integer. These folks don't like the idea of measuring a sphere as "Using a sphere precludes choosing an integer at all, because of the irrationality of p." On the other hand, if the measurement process goes better with a sphere, I'm all for that. (of course, there are still an integral number of atoms in a sphere of just like there are in a cube)

I guess everything has to start somewhere. Avogadro's number depends on the precision of the definition of a gram, since the mole defined via the gram.

All of our "constants" depend on each other, since they are all defined in relation to each other.

It seems like the best way to measure and define it would be to measure the lightest stable element (say, hydrogen) and the heaviest (say, lead) and average the two? Kind of like measuring 100 paperclips and 100 quarters. And then counting how many paperclips or quarters make up a pound. It would seem like the correct number would emerge pretty quickly.

(And then there's the issue of rounding. If we round Avogadro's number so it's easier to calculate, does that throw off some other definition? I say fuck it, pi is 3, avogadro's number is 6 x 10^23, speed of light is 300,000,000 m/s, and all science math is done in base 8 from now on. We'll get to Mars in no time, with all that math made easier!)
posted by gjc at 12:19 PM on June 8, 2013 [1 favorite]

Why are they using silicon? It is only the second most pure crystal they can make: High purity germanium would be better, wouldn't it?

From what I recall of a New Scientist article I read a decade ago, silicon was chosen because we have the most in-depth knowledge of working with silicon at an atomic scale, thanks to the silicon chip industry.
posted by YAMWAK at 12:23 PM on June 8, 2013 [2 favorites]

Why are they using silicon? It is only the second most pure crystal they can make: High purity germanium would be better, wouldn't it?

According to Wikipedia, "Silicon was chosen because a commercial infrastructure with mature processes for creating defect-free, ultra-pure monocrystalline silicon already exists to service the semiconductor industry."
posted by hypersloth at 12:26 PM on June 8, 2013

Yeah, in order for even a basic chip to work it needs 'nine nines' of purity, literally 99.9999999% purity
posted by delmoi at 2:32 PM on June 8, 2013

If you define k in terms of the mass of Si and avogadro's constant, then you risk making the weight of 1 mol of C something other than 12g; if you define it in terms of C per mol but measure Si then your definition of k is no better than the measurement of the ratio of the Si and C masses (This is an experimental quantity rather than a quantity that comes from calculation, right?). I guess it wouldn't be that bad to make 1 mol of C weigh something other than 12g, since it would differ only in the micrograms.
posted by jepler at 4:33 PM on June 8, 2013

Jepler, while we don't know the exact mass ratio of C-12 and (the various isotopes of) Si, we know it to a very high precision: 12 or 13 significant digits in weighted aggregate over the occurring isotopes, I think. While this is nowhere near the most sensitive mass measurements we can make, it does make the fifty or so micrograms talked about in that video look absolutely gigantic. So if we pick up a 1e-13 or so error relative to the current kilogram, that's not a big problem because it's extremely likely that the current kilogram has changed by many orders of magnitude more than that over recent history.

The goal is to very accurately measure NA based on the current kilogram (since we know the lattice constant of Si very precisely as well) and then we don't have to rely on the standard kilogram: we can then go from NA to a physical kilogram any time we like. Assuming we like lots and lots of spherical polishing and measuring.
posted by introp at 8:53 PM on June 8, 2013

YAMWAK and delmoi: Sorry for the late response. My understanding is that we can make germanium crystals even more pure, which is why really good gamma-ray detectors use them (high purity germanium detectors, or HPGe) while cheaper ones use silicon (as it doesn't cost \$40,000 for the crystal needed to run it).
posted by Canageek at 10:30 PM on June 14, 2013

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