Moorday: how is the Avogadro constant measured?

October 23 is Moore Day, an unofficial holiday, but it is also popular among chemists and chemical enthusiasts. This day commemorates the Avogadro constant and the unit of measure of the important substance Moore.

in high school chemistry, the Avogadro constant is often used to calculate. One mole of matter contains Avogadro constant molecules, and the approximate value of this constant is 6.02 × 10 ^ 23. So people commemorate it at 6:02 on October 23rd.

so how did you get the Avogadro constant? In history, there are many methods of measurement, such as the Brownian motion method, oil drop method, electricity analysis method, but now, to get the most accurate value, the X-ray crystal density method is used.

the key to this method is to make a very round, perfect ball first. As shown in the picture:

the ball is composed of high purity silicon-28 crystal and weighs a full kilogram. It takes a lot of work to make it, first of all, to get high-purity silicon crystals, and then after accurate calculation and polishing.

so how to measure the Avogadro constant? To put it bluntly, it is to count atoms. The lattice arrangement of the silicon sphere is very regular, and the atomic weight and total mass are determined, so as long as the number of atoms is calculated from the lattice parameters, the Avogadro constant can be calculated. By the way, the last published calculation of the Avogadro constant is 6.02214082 (11) x10 ^ 23.

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Why do I need to measure this constant very accurately? One reason is that scientists have to redefine mass with precise physical constants. For a long time, the method of defining a unit of mass has been simple and crude, which is to cast a piece of platinum-iridium alloy and then set its mass as a "kilogram" standard, and all other instruments and things will be calibrated with it. This platinum-iridium alloy is the kilogram original, and its original is stored in the International Metrology Bureau in Paris, France, and has been used for more than 100 years. This is what it looks like:

it can be seen that to keep it stable, people are isolated from the outside air during preservation, and the platinum-iridium alloy itself is a very stable material. So far, the mass of the kilogram original has not changed-- it sounds like OK?. But as an important standard, it is not possible to guarantee only the "basic". Its quality has changed, although it is only a few dozen micrograms, it will still have a lot of impact on situations where precise measurement is needed.

so what to do? What the scientists think is that we don't need the physical object and redefine the mass through the physical constant. As long as the constant is measured accurately enough, it can be stable all the time.

at present, what is determined to define the mass is the Planck constant, which sounds like it has nothing to do with the silicon ball. Measuring the Avogadro constant is a way to calibrate the Planck constant, which can help scientists check whether the new standard of quality is accurate.