Up until the 18 century , only a few elements were known to man . Some of them were known already since ancient times , like iron , gold , copper and sulfur . In the 18 century , the pace of discovery accelerated . Scientists began to search for order in the properties of the various elements , and tried to classify them into groups , exposing their common properties . The breakthrough came thanks to the work of Dmitri Mendeleev , a Russian chemist who , in 1869 , published the periodic table . By that time , 63 elements had already been discovered . Mendeleev , prepared a card for each element , on which he wrote the element’s name and its properties . When he arranged the 63 element cards in ascending order according to their atomic mass , he discovered that the elements’ properties recur cyclically : every few elements , another element occurs whose properties are similar to an element that preceded it . The picture below shows the modern periodic table , which is based on Mendeleev...  אל הספר
מטח : המרכז לטכנולוגיה חינוכית