The mole, symbol mol, is the International System of Units (SI) unit of the amount of substance which is the quantity referring to a measure of the number of specified elementary entities, such as chemical elements or compounds in a sample. In the current SI, the mole is defined by specifying the mass of carbon-12. But this base unit is not an invariant of nature because the mass is defined by the material artefact. According to efforts to define the base units in SI using true invariants of nature, the mole will be redefined by fixing the numerical value of a fundamental constant, the Avogadro constant. In the new SI, the definition of the mole can be realized through the experiments that lead to the determination of the Avogadro constant. The best experimental value of the Avogadro constant has been obtained by the X-ray crystal density experiment using silicon-28 highly enriched silicon sphere in the frame work of the International Avogadro Coordination. In this paper, the current definition of the mole and practical aspects of this unit are introduced, then the principle and technical challenges in X-ray crystal density experiment for redefinition of the mole are discussed.

The International System of Units (acronym: SI) is founded on seven base units (meter, kilogram, second, ampere, kelvin, mole, and candela) corresponding to seven base quantities (length, mass, time, electric current, thermodynamic temperature, amount of substance, and luminous intensity). SI was formally established in 1960 by the 11th CGPM. It has been revised from time to time in response to requirements of users and advances in science and technology. However, the most significant revision is going to be done in November 2018 by the 26th CGPM. Four base units (kilogram, ampere, kelvin, and mole) will be given new definitions linking them to exactly defined values of Planck constant, elementary charge, Boltzmann constant, and Avogadro constant, respectively. In this paper, historical background for the revision of SI is described and scientific principle of redefinition is explained. The procedure used to redefine meter from the speed of light in a vacuum is used as an example. After this revision, uncertainties of many other fundamental constants will be eliminated or reduced. From May 20, 2019 (World Metrology Day), the revised SI will come to practice.