![]() ![]() This implies that by current measurement conventions, the mass of a single atom of carbon-12 is changing in time, whereas modern theory postulates that it remain constant. It is estimated that Le Gran K may have changed about 50 micrograms-that is, roughly by about 150 quadrillion (1.5 × 10 17) atoms-since it was constructed. These changes cannot be measured exactly, simply because there is no "perfect" reference against which to measure them- Le Gran K is always exactly one kilogram, by definition. Periodic cleanings and calibration measurements result in abrasion of platinum-iridium and accretion of cleaning chemicals. The problem is that the mass of Le Gran K is known to be unstable in time. Originally the mass of one cubic centimeter of water at exactly 3.98 degrees Celsius and atmospheric pressure, for the past 117 years the definition of one gram has been one-thousandth of the mass of " Le Gran K," a single precious platinum-iridium cylinder stored in a vault in Sèvres, France. This definition of N A and the current experiments to estimate it, however, both rely on the precise definition of a gram. According to the National Institute of Standards and Technology (NIST), the current accepted value for N A is: The current state of the art estimates the value of N A, not based on experiments using carbon-12, but by using x-ray diffraction in crystal silicon lattices in the shape of a sphere or by a watt-balance method. Today, Avogadro's number is formally defined to be the number of carbon-12 atoms in 12 grams of unbound carbon-12 in its rest-energy electronic state. ![]()
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