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I have some friends that are talking about why a silicon ball with an exact number of atoms is a good/bad measurement of mass (1kg) and things are getting pretty exotic. Is there a layman's explanation for each side of the debate?

EDIT: I got my friend that thinks exact number of atoms is not a good method to boil his side of the argument down to:

We know the mass of a system is not the sum of the masses of its constituent parts, so specifying the kilogram as the mass of N constituent atoms doesn't work if we measure the mass of the system containing them.

The side which said it is good basically says that it is very accurate and a reproducible reference.

Bill N
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  • How can you expect a comment on each side of the debate is you have not disclosed either side of the debate? – paparazzo Jul 28 '15 at 18:11
  • Is your question why the definition of the kilogramm is changing from the mass of a precisely defined body to a definition based upon say watt balance? – Bort Jul 28 '15 at 18:35
  • @Frisbee One side says the silicon ball is a good way to measure 1 kg, the other side says its not for reasons. If you want to me to post what the reasons are, I can't do that, because I don't understand said reasons. So I'm asking for a digestible version of said reasons –  Jul 28 '15 at 18:49
  • @Bort No, to rephrase the question, what are the pros and cons of making a silicon ball with an exact number of atoms the definition of 1kg? This is assuming that its going to be more accurate than the slab of platinum that's stored in France –  Jul 28 '15 at 18:52
  • @ton.yeung ah okay.. so my first answer would be: pro: We have the technology to handle silicon very, very, very well (from computer industry) and it couples the definition to avogadros constant which is nicely controlable. con: weighing still is cumbersome and has a high error, using ideas like watt balance we can link the kilogram to very precisely known quantities like plancks constant – Bort Jul 28 '15 at 19:02
  • @Frisbee added clarification on each side of the debate –  Jul 28 '15 at 21:03
  • Last night a friend of mine told me that he thinks Casablanca is a really good movie. Can you tell me why he thinks that? – WillO Jul 28 '15 at 21:07
  • @WillO 5 minutes before you posted the comment I had clarified what each side had to say. If you have some other point, I'm not sure what it is. If you're actualy serious about wanting to know more about Casablanca you should try asking at http://movies.stackexchange.com –  Jul 28 '15 at 21:26
  • The goal is not to create a 1 kg silicon sphere with an exact number of atoms. That is an impossible task for the foreseeable future. – David Hammen Jul 28 '15 at 23:28
  • ton.yeung: As @Shaurya Bhave's answer indicates, the more pertinent question might be to ask, whether and why it is a bad idea to attribute some particular value of "mass" so some (non-zero) number of artefacts (concretely: to attribute a "mass of 12 grams" to the Avogadro number of C12 atoms. (And I'd say that's bad because it precludes asking and measuring whether two exemplars of "C12 atoms" had equal "mass", or whether the "mass" of one particular exemplar of "C12 atom" had remained constant, or not.)). Anyways: +1 to your question. – user12262 Jul 29 '15 at 05:46
  • Related question: why is the prospective new kilogram standard a sphere – Floris Aug 04 '15 at 15:39

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Con: There are 3 naturally occurring isotopes of silicon, so isotopic purification would be required after elemental purification. I'm unsure about how hard it is to maintain pure silicon.

Isotopically, beryllium would be better (100% $^9$Be, naturally), but I don't know about its reactivity, either.

Carbon has two naturally occurring isotopes, but we know a LOT about carbon chemistry.

The Wikipedia article on the kilogram has a wealth of information about the new Watt balance, as well as discussing $^{12}$C and silicon sphere standards.

Bill N
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    Regarding your con, the silicon spheres in question are 99.9995% silicon-28. Achieving this was no mean feat. Getting that level of purity played a big part in the $3.2 million cost of each of these silicon spheres. – David Hammen Jul 28 '15 at 20:35
  • I just added the kilogram link from Wikipedia. Interesting. – Bill N Jul 28 '15 at 20:47
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The object of making the silicon sphere was not to define the kilogram by the mass of sphere, but redefine Avogadro's constant in terms of the number of silicon atoms. Then use the fixed Avogadro's constant to fix the kilogram. Avogadro's constant was defined as the number of atoms in 12 grams of carbon atoms. You can read up on the Avogadro project here https://en.wikipedia.org/wiki/Kilogram#Avogadro_project

Shaurya Bhave
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