3. Periodic classification of the elements

3.1 Historical classification

The first systematic classification of chemical elements was based on the analogy of the properties of certain chemical elements. Mendeleïev classified elements in rows (corresponding to today's columns) when their chemical properties were similar. In a periodic manner, he placed them on his classification taking into account their increasing molar atomic mass, ignoring the notions of protons, electrons or neutrons. Mendeleïev had to leave some cells empty, as some elements were still unknown at the time, but he felt that these cells should correspond to elements whose properties he had even foreseen. As it turned out, he was right: all the empty cells were filled, and an additional column was added to include the noble and inert gases discovered only...