New, Ultra-Precise Gauge Could Facilitate Redefine the Kilogram

at 16 July 2015

A new, enormously precise measure of Avogadro's number, an elementary constant, could make sure solid foothold for a new description of the kilogram that does not rely on a single hunk of metal sitting in France.

Every junior high school chemistry student learned Avogadro's number, or 6.022 X 10 ^23, a enormous value. Because Avogadro's number defines how many atoms or molecules are in a mole of matter, each mole of a material weighs a different amount depending on the substance in question. So, a single mole of water would weigh just 0.56 ounces (16 grams), whereas a mole of lead would weigh about 7.3 ounces (207.2 grams).

Scientists have tried to accurately measure Avogadro's number in the past, each time using a single silicon ball that weighs 2.2 pounds (1 kg). Because silicon crystallizes into a lattice with eight atoms for each repeating unit of the lattice, they can acquire at Avogadro's number by measuring the volume of each cubic cell. By knowing the ratio between the volume of the crystal and the volume each silicon atom occupies, the team can then presume how many atoms are in the tiny sphere.