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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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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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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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