Check out the video if you have ever used a “kilogram” to measure something.
The “reference kilogram” is a lump of metal in France, the kilo being the only metric measurement not based on some objective standard. A project is underway to replace the kilo with something independently reproducible: perfectly spherical balls of based on Carbon-12. Incidentally, these new kilos would also be “the roundest objects in the world.”
This is brilliant. Fabrication of a Silicon Sphere By definition, an Avogadro number of Carbon-12 atoms weigh exactly 12 grams. As such, the kilogram could be defined as the mass of 1000/12 * Avogadro’s number of Carbon-12 atoms. The Avogadro constant itself is obtained from the ratio of the molar mass to the mass of an atom. For a crystalline structure such as silicon, the atomic volume is obtained from the lattice parameter and the number of atoms per unit cell. The atomic mass is then the product of the volume and density.