Mendeleev and the Periodic Table

Johann Wolfgang Döbereiner

Ever since the time that individual elements were discovered, scientists have been trying to find a logical way to organize them. By 1869, a total of sixty-three elements had been discovered. As more and more elements were discovered, scientists began to see that there were similarities between some of them. The periodic table began in 1829 when Johann Wolfgang Döbereiner attempted to organize the chemical elements using their physical properties. Döbereiner noticed that there were triads of elements that all showed similar chemical properties. What exactly is a triad? In chemistry, any of several sets of three chemically-similar elements, the atomic weight of one of which is approximately equal to the mean of the atomic weights of the other two. Döbereiner noticed the triads of chlorine-bromine-iodine, calcium-strontium barium, and sulfur-selenium-tellurium. These triads were the earliest classification of the elements. However, since this classification was based on atomic weight, it is inconsistent.

Dmitri Mendeleev

One of the early tables was arranged into triads because there seemed to be many groups of elements which naturally had three members. Another table was arranged in eight distinct groups called “octaves”. It was Dmitri Mendeleev who is given credit for being the Father of the Periodic Table. Dmitri noticed patterns in the elements and their properties. He created a card for each element and listed their properties. He used these cards like pieces of a puzzle, arranging and rearranging them according to their atomic weights. From this work, he developed his table and published the results in 1869. Why was his table accepted over the others? Using scientific theory and some rather complicated thinking, he recognized that the atomic weights for some elements were incorrect and he moved them in his chart. Most spectacularly, he left spaces in his chart for undiscovered elements. The holes that were left in Mendeleev's chart were given the names of "eka-silicone" and "eka-aluminum." Later, the elements gallium and germanium would fill these holes. He even predicted many of their properties that were later proven to be correct. Mendeleev actually arranged his table so that elements with the most similar properties were side by side.

Identifying the Elements Review

Now that you have explored the history of the periodic table, review your knowledge in this non-graded activity. Read each question and decide whether it is True or False. Then, click SUBMIT to check your response. Click on the interactivity thumbnail, and then click NEXT to get started.