Periodni sustav elemenata (PSE) ne može se do kraja razumjeti ako se ne sagleda njegov razvoj, prije i poslije Mendeljejeva. U radu je ukazano na komplementarnost prvotnoga sustava (temeljenog na relativnim atomskim masama) i suvremenog sustava, temeljenog na naboju jezgre. Predočavanje povijesti razvoja PSE može poslužiti i kao uvod u metodologiju znanstvenoga rada, a svakako pomaže razumijevanju osnovnih kemijskih pojmova (atomska masa, protonski broj, element, elementarna tvar)The teaching of the periodic system in the historical context has an advantage because students in this way comprehend more easily the basic chemical concepts, such as relative atomic mass, proton number, chemical element, elementary substance, equivalent and molar mass. The story of the periodic system also shows the following: (1) Before a key scientific discovery there is a long period of data collection and systematization, (2) Theories in chemistry must be based on experimental results and methods, (3) The advancement of science is also determined by the development of concepts, and (4) Scientific theories usually do not contradict but support each other. Therefore, the historical periodic system (based on relative atomic mass) is not obsolete, but complementary to the modern
