Atoms and molecules in our daysFor a long time the chemists have some difficulties to understand the differences between atoms and molecules. In the 1860, in Karlsruhe (Germany) an international congress of chemists tried to resolve these difficulties and to find an unique chemical symbolism. To this conference Stanislao Canizzaro took part with his relation in defence of the Avogadro's principle, exposing the just system in order to determine the atomic weights and molecular. In 1811 Avogadro published the "Essai d'une maniere de determiner les masses relatives des molecules elementaires des corps, et les proportions selon les quelles elles entrent dans ces combinations"; in this work he introduced a strong difference between the concept of atom and the concept of molecule. In the 1814 Avogadro expressed his principle in this way: .....The same volumes of different gaseous substances have
same numbers of molecules, if the temperature and the pression are the same;so
the densities of different gases measure the masses of their molecules and the
proportion of volumes, in the gaseous combinations, are like the proportion
between the numbers of molecules that react to form other composts.... After Canizzaro's relationship the following proposal:"We propose to adopt different concepts for molecule and atom: we consider molecule the smallest amount of substance that enters in relation and that conserve the physical characteristics, and we mean for atom the smallest amount that enters in the molecule of its compounds ". The name "number of Avogadro" is just an honorary name given to present the deliberate value of atoms and molecules in a particular amount of matter that the chemists call MOLE of a chemical substance. The best current experiments give this value: |
