Il nuovo Sistema Internazionale di Unità di Misura (SI)
La nuova definizione della quantità di sostanza e della mole
The article was published by Giuiano Moretti and Ida Pettiti in the magazine of the Società Chimica Italiana: La Chimica nella Scuola
Starting from May 20th, 20 I 9 the new International System of Units (SI) should be in use in all the Nations. In this new SI the seven base quantities are defined by means of seven constants of nature with fixed numerical values. From a practical point of view the base quantities and the base units are the same of the previous SI but in the new SI it is realized a system free from physical units based on artifacts. The mass unit, the kilogram, is no more based on a platinum-iridium ingot maintained at the International Bureau of Weights and Measures (BIPM) in Sèvres (France).
In the present contribution we report the definitions of the seven base units and the numerical value of the seven constants of nature by which they are characterized. We devote particular attention to the amount of substance (chemical amount) and to its base unit, the mole, which are of fundamental importance far Chemistry, as well as the Periodic Table of the elements.