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.