It was important that Mendeleev took Te and I (Tellurium and iodine) because if he mixes them the wrong they can explode. Iodine ALWAYS has to be placed AFTER Tellirium if they are being mixed, because Iodine has lower relative atomic mass than Tellirium. Meaning that if you do mix it in the right order (iodine THEN Telliurim) the result will just be little sparks. But if you them wrong, the result is an explosion.
Yw. :)