Why couldn't mendeleev organize the entire periodic table durning his rea search

The answer is :

Some elements were undiscovered at this time.

The explanation:

- Mendeleev depended on the atomic mass of the element in his organization of the entire periodic table , and this made him to put all  the elements which have the same chemical and physical properties with each other into vertical column called groups

- and this method created a mean of a gaps in the rows, he assumed that those gaps represent the undiscovered elements ,but the new periodic table is organized by the increasing in atomic numbers.  

the dependant variable is spilled water

the independent is the heat

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